Although your mileage may vary (depending on the rep you get at the store), in general, the answer is yes: you can walk into an Apple store, have them ring up an AT&T model as "device only", and that phone will be immediately registered in Apple's system as unlocked, and they won't require you to open a new AT&T account or associate that purchase with an existing account.
This has worked with the AT&T models of iPhones ever since Apple started selling the iPhone 4 unlocked in the U.S. via Apple Stores. Apple has an unlocked GSM model with a separate SKU, but they specifically set up the system so that the employees at the store can ring up an AT&T model as an unlocked model instead. This was done so that in the event someone wants to buy an unlocked phone and a particular Apple store has run out of stock of the unlocked model, they can substitute an AT&T SKU in its place.
Officially, with the iPhone 5, there is no unlocked SKU yet, and I think that officially, they aren't supposed to sell you a phone without getting account information from you. But if you can talk an Apple Store employee into selling you an AT&T iPhone 5 as "device only" (no activation, full price), it will be unlocked since all AT&T SKUs, including the iPhone 5, are set up this way. (This same thing happened with the iPhone 4S back when it was released...there was a period of a month or two between when the 4S came out and when Apple started selling unlocked versions of it officially, but people figured out that the selling-AT&T-SKUs-as-unlocked policy carried over to the 4S part numbers as well. I managed to buy an unlocked 4S this way before it was officially available. In my case, I got 0 pushback from the rep who helped me...I was in and out in like 5 minutes. But I've heard other stories of people who have tried to do this at other stores and being told that they aren't allowed to do that. It's technically possible...you just have to convince the person who is helping you to actually ring it up that way.
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Note that this system only works when you make the purchase at an Apple store. If you purchase a phone from AT&T retail at full-price, the phone will still be locked. However, according to AT&T policy, if they didn't subsidize the phone purchase, they will unlock it for you upon request. This wasn't true when the 4S was released, so back then the Apple store was the ONLY way to get an unlocked one before they were officially available. With the iPhone 5, it's simply less of a hassle buying directly from Apple since the phone's serial # is immediately flagged as "unlocked" in Apple's system, but if you have no Apple retail store near you, you should be able to buy from AT&T at full-price and then request an unlock from them.
-- Nathan
