Rather than making a custom back as described at Wishlist:Expansion_Back, spacers could be used instead. They would be snapped onto the phone in place of the back, and then the back snapped onto the last spacer. The user could add as many spacers as desired.
Very crude artist's conception of a spacer:
One spacer being inserted between the phone and its back:
One spacer already snapped into place:
Two spacers snapped into place:
For aesthetic reasons, the thickness added by each spacer could be made identical to the width of the phone's long narrow "side panels" that hold the USB connector, the earpiece jack, and the two side buttons. (I think this is several millimeters -- anybody got a more precise measurement?)
The spacer could be made from the same material as the body of the phone, so the color would match. Only the edge of the spacer is visible after the phone is reassembled.
Manufacturing such a spacer would require a simple two-piece injection mold (perhaps?). If the expense of injection molding cannot be justified, they could be carved from a sheet of material (almost any carveable material, including wood) with a small CNC machine.