If you paid full price for the phone, I would think AT&T has no say in the matter. It's the same as buying a no-commitment officially unlocked phone from Apple. This is not unintentional; when my friend picked up my last 4S on early upgrade pricing (a fact I was not aware of until recently), the phone was still locked to AT&T.
However, when I walked in today and asked for an off-contract phone in exchange for my early upgrade phone, the store rep actually seemed confused. He even asked if there was any specific reason I wanted to exchange the phone and reiterated to me that the unsubsidized phone would be locked to AT&T. I took the chance and found out that when you buy an off-contract phone directly through Apple (I think this is key; AT&T stores still sell locked phones at full price) it will be unlocked.
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To anybody wondering about this: I went in to an Apple Store today to exchange my phone that was purchased at an early upgrade price for an off-contract phone. When I left the store and called AT&T to cancel the new contract started with the previous purchase I was informed that as soon as I returned the phone my contract went back to it's prior status.
So basically, if you return your AT&T subsidized phone within the return period, your contract should go back to its previous status automatically.