CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 60/599,419, filed August 6, 2004 and entitled "Personalizing Luggage," and U.S. Application No. filed July 20, 2005 entitled "Personalizing Luggage", which claims priority to U.S. Provisional Application No. 60/599,419, filed August 6, 2004, both of which are incorporated by reference herein.

TECHNICAL FIELD This invention relates to luggage and other personal, totable or wheeled containers for transporting personal effects.

BACKGROUND Commercial luggage is available in several styles and colors. Typically, a consumer selects a color and style based on personal preference at the time of purchase, and if tastes later change, purchases additional luggage.

SUMMARY Various aspects of this invention feature luggage, such as a suitcase, or other containers for a user to personally transport his or her personal effects, with one or more outer panels selectively removable and replaceable with one or more panels of similar construction but with different outer appearance, to alter the visual appearance of the luggage or container. According to one aspect of the invention, luggage includes a main frame defining an internal compartment for items to be packed and transported, and at least one outer panel secured to the frame and forming an outer surface of one side of the luggage. The panel is releasably secured to the frame for replacement with any of a number of interchangeable panels of differing visual or tactile properties. Preferably, the outer panel extends across a majority of one side of the luggage, or more preferably, across substantially all of one side of the luggage. In many constructions, the frame includes a sub-panel forming an interior surface of the luggage and covered by the outer panel. In some cases, the sub-panel defines slots into which tabs of the outer panel are releasably secured. The outer panel tabs may define apertures sized to accept respective fastening clips disposed on a side of the sub-panel opposite the outer panel, to hold the outer panel to the sub-panel. Such fastening clips, in some embodiments, include springs compressed between an inner surface of the sub-panel and an opposing surface of their respective tabs. Each spring may define a recess sized to receive a feature of the tab when the clip is fully engaged. In some cases, each spring clip includes a pair of spring fingers with outer, oppositely directed cam surfaces for engagement with the tab as the spring fingers are inserted through the tab. The spring clip may also have an inclined surface that progressively engages the tab as the spring clip is pressed into full engagement, to draw the outer panel against the sub-panel. In some cases, the outer panel has projections extending from its inner side, and the frame defines respective holes arranged to receive the outer panel projections. Some such examples also include removable clips releasably engaging the outer panel projections with the projections extending through the frame holes, to secure the outer panel to the frame. Such projections may be mushroom-shaped, with each clip including a pair of spring fingers with opposing cam surfaces configured for engagement with a stem portion of its respective projection. Each clip may also include a third spring finger resiliently deflectable in a direction perpendicular to a normal deflection direction of the pair of spring fingers and arranged to engage an upper surface of a head of the mushroom-shaped projection. The third spring finger may also have a nib projecting toward the pair of spring fingers and arranged to be received in a recess defined in the upper surface of the projection head. In some cases, the spring clips are caps pressed over distal ends of their corresponding projections. In an illustrated example, each projection includes an outer lip extending radially from an outer surface of the projection, each projection also defining a slot along one side, enabling resilient collapse of the projection. In such cases, the frame holes are preferably sized to require temporary collapse of the outer panel projections during attachment of the outer panel to the frame. The cap, in some configurations, defines an inwardly extending rim positioned for engagement with the outer lip of the projection when the cap is secured to the projection. The cap may also have a central nib extending from its inner surface, the nib of a size selected to cause the outer lip of the projection to be forced outward into secure engagement with the rim of the cap as the cap is pressed onto the projection. In some examples, the outer panel is releasably secured to the frame by fasteners accessible from inside the luggage. In some cases, the outer panel carries a graphic image visible from outside the luggage. In some configurations, the frame includes a hinged side openable for accessing contents of the luggage, for example with the outer panel disposed on the hinged side of the luggage. Another aspect of the invention features a method of personalizing luggage. The method includes removing a first outer panel from a side of a piece of luggage, and securing a second outer panel to the side of the piece of luggage from which the first outer panel was removed, effectively replacing the first outer panel with the second outer panel, where the second outer panel is of a different outer visual appearance than the first outer panel. In some instances, the removable panel is releasably secured to a frame of the luggage or container by one or more releasable fasteners that are configured to reduce the chance of inadvertent release. Other aspects of this invention feature removable spring clips and fastening systems as shown and described herein, which may be employed as releasable fasteners in several applications beyond personal luggage. The modular luggage construction approach disclosed herein permits personalization and customization by interchangeable appearance covers, by interchangeable lids and by interchangeable primary containment perimeters, so that the luggage can be selectively constructed with either a hard or soft upper containment perimeter and lid, and so that aesthetic or tactile treatment of the lid can be changed very easily by means of a unique fastening system. In some cases, the lower containment perimeter is affixed to the chassis by custom fasteners that allow release only by a tool provided with the system, while the outer appearance shell is affixed to the lid by a perimeter fastening system that securely adheres the appearance shell but permits fast removal, either with a special tool provided with the system, or in some configurations without tools. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS Fig. 1 is a perspective view of a side panel of a suitcase, otherwise shown in dashed outline, and a replaceable cover for the side panel. Fig. 2 is a cross-sectional view through an edge region of the side panel, with the cover secured. Fig. 3 is a perspective view of a first cover fastening means. Figs. 4 and 5 are cross-sectional views through the spring clip of Fig. 3, during and after engagement, respectively. Fig. 6 is a perspective view of a second cover fastening means. Fig. 7 is a cross-sectional view through the spring clip of Fig. 6, after engagement. Fig. 8 is a perspective view of a third spring clip. Fig. 9 is a cross-sectional view through the spring clip of Fig. 8, after engagement. Fig. 10 is a perspective view of a fourth cover fastening means. Fig. 11 is a perspective view of the cover projection of the fastening means of Fig. 10. Figs. 12 and 13 are cross-sectional views through the spring clip of Fig. 10, during and after engagement, respectively. Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION Referring first to Figs. 1 and 2, a suitcase 1 has a sub-cover 6 forming one side of the suitcase, and an interchangeable, releasable cover 2 that can be secured in place to substantially cover sub-cover 6. Cover 2 has integral, apertured tabs 4 that protrude thru respective slots 8 in the sub-cover, and are secured by fastener clips 10 inserted through the apertures in the tabs on an opposite side of sub-cover 6. Cover 2 is one of a set of covers of different outer styles and/or colors, any one of which may be selectively secured to sub-cover 6 to alter the outer appearance of that side of suitcase 1. Although only one cover is illustrated, covering a single side of suitcase 1, it will be understood that the other sides of the suitcase may be equipped with similar sub-covers and replaceable covers. In the embodiment shown, the structural integrity of suitcase 1 is provided by the sub-covers or other framework, and not by the interchangeable covers. In this manner, the majority of the bulk of the suitcase remains relatively unaltered by cover replacement, and can be readily fashioned by known methods out of known materials, without significant cost increase over conventional suitcases of comparable size. The sub-covers or other framework define the internal compartment. In the embodiment shown in Fig. 1, sub-cover 6 forms the hinged lid of the suitcase. One version of the fastener clip 10a, shown in Fig. 3, is a spring clip of plastic or other material molded, extruded or otherwise formed in an arc. The clip defines a ribbed recess 12 for thumb engagement, and a slot 14 in the outer surface of the clip at the top of the arc that receives an inner surface of the apertured tab 4 of the outer cover as the clip is slid through the aperture of the tab on an inner side of sub-cover 6, as shown in Figs. 4 and 5. Clip 10a is formed to engage the inner surface of the sub-panel at relatively wide, planar contact areas so as to provide a substantial clip retention force without overly stressing the sub-panel. In this example, sub-panel 6 includes a raised rib segment 16 (Fig. 3) adjacent and perpendicular to slot 8, to help block the spring clip from inadvertent compression by forces applied, for example, by shifting suitcase contents. Another version of the fastener clip 10b, illustrated in Figs. 6 and 7, is a three prong snap fastener that both snaps and wedges in place within the apertured tab 4 of the outer cover, to secure the cover to the sub-cover 6. Clip 10b is injection-molded plastic, with two outer spring fingers 18 defining lateral cam surfaces that engage opposing inner surfaces of tab 4, resiliently deflecting fingers 18 toward wedge finger 20 as the clip is pressed into place. When fully engaged, spring fingers 18 snap outwardly to secure the clip to the tab. A sloped upper surface 22 of wedge finger 20 progressively engages an underside of tab 4 to draw the outer panel securely against the sub-panel. Clip 10b is released by manually pinching the distal end of the clip to compress spring fingers 18 toward one another until their cam surfaces clear the sides of tab 4. A third version of the fastener clip 10c, illustrated in Figs. 8 and 9, is a two stage snap clip with two opposing lateral spring fingers 24 and an elevated spring finger 26 that deflects in a direction perpendicular to the deflection direction of lateral spring fingers 24. Elevated finger 26 has a downwardly projecting nib 28 near its distal end. Clip 10c is configured to releasably engage a mushroom-shaped projection 30 extending from the inner surface of the outer cover 2, as shown in Fig. 9. As the clip is forcibly slid into place along the inner surface of the sub-panel 6, inner cam surfaces of spring fingers 24 variably engage the stem of projection 30 while upper surfaces of spring fingers 24 engage the underside of the head 31 of projection 30 to secure the outer panel to the sub-panel. As clip 10c is pushed into full engagement, nib 28 of the elevated spring finger 26 is received within a recess 32 defined in the upper surface of the outer panel projection 30, providing additional clip retention means. A fourth releasable fastener clip 1Od and mating projection 34 are shown in Figs. 10-13. Clip 1Od is in the form of a hollow molded cap with an inwardly projecting inner lip 36 that snaps over an outer lip 38 of projection 34. Cap 1Od is axisymmetric about the vertical axis of projection 34, while projection 34 is split along one side to allow the projection to resiliently collapse as the cap is pressed through a corresponding hole in sub-panel 6. As shown in Figs. 12 and 13, during engagement of the cap onto the projection 34, a downwardly projecting, conical nib 40 at the center of the inner surface of cap 1Od forces the outer lip 38 of projection 34 outward into secure engagement under the inner lip 36 of the cap. To remove the outer panel 2, the caps 1Od are pried, pulled or otherwise released from their projections 34, which are then manually compressed, such as by pinching between fingers, for withdrawal through the holes in the sub-panel. Replacement outer panels can be provided with the luggage or as after-market accessories sold separately. A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.