I personally am not returning the phone (I love my iPhone 4 mucho), but people have asked me how they go about it if they want to go back to their 3GS or switch carriers all together. Can someone please explain how it works?
I can't see why it wouldn't revert back to your old contract if you return your phone. No upgrade, no subsidized price benefit used, no new contract. Makes sense to me. I'd confirm with AT&T first.