There was much discussion of this when the iPhone was released and the consensus was that you certainly could keep the phone. You wanted a new AT&T contract and free phone -- you buy it, then you decide to give yourself a hardware upgrade. Because it is an iPhone, it resets your 2 year contract but you certainly can keep the phone.
If it was 1 month later, would you be required to return the original phone -- how about if it were 1 year later? The answer to both is no -- you just start the 2 year contract over.