Based on this teardown, looks really easy to pull off the back glass cover. Once the supply chains make available service parts, we could easily add a white rear cover. I personally like the rear of the white iPhone, but not the front, prefer it in black.
http://www.ifixit.com/Teardown/iPhone-4-Teardown/3130/1
http://www.ifixit.com/Teardown/iPhone-4-Teardown/3130/1