Returned my 4S today, buying a white one next week. The guy who was doing my return had a brand new black 4S with a white back! He said if I wanted a white one with a black back, go on Amazon and buy one. He said Amazon sells official Apple ones. He also said replacing the back does not void warranty. But if my phone needs service, he said to just put the white back on. He then asked 4 other blue shirts, and they said it does not void warranty by unscrewing the screws and replacing the back.