I don't think it was an accident that the full price iPhone from AT&T came unlocked. I doubt that they'd let customers slip through the crack like that because AT&T wants money. Secondly, AT&T never relocked the phones. So my guess that if you get the full price AT&T iPhone it would be unlocked. Now whether the Apple store carries that model anymore I don't know. Just my 2 cents.
With that being said, I personally would go for the factory unlocked iPhone just for peace of mind, but I didn't wait for it when it launched so I actually have the AT&T version hehe