I have some scratches on the back of my phone already, whatever it is made of isn't seem as scratch-proof as the glass (?) back of the 4/4S.
And speaking of the iPhone 4, I was quite surprised that Best Buy still offered to buy it back for $120. I thought the 4S would fetch that much in stores like BB that lowballs you but not the 4. I know I could probably get more on ebay or something but selling to BB is pretty hassle-free and no risk of being scammed and stupid ebay and paypal fees so I might sell it to them.