why fret over this? as soon as Apple does release the white iPhone I will simply go to an apple store and order the white rear glass and have a genius install it...and no it won't void the warranty, I have spoken to my favorite Apple genius and this is what he say most Apple employees will do
If all you care about is the white back then buy the 15$ one online at multiple stores. You can switch out your back one if you ever have any warranty issues. It's only two screws.
The rest of us want a white front AND back.