As of today, you no longer need an AT&T plan to purchase an iPhone at an Apple Store. Apple’s new policy is that anyone can walk into an Apple Store and pick up an iPhone 8GB for $499, 16GB for $599 and 32GB for $699. They will allow one unit per customer per day up to 10 iPhones.
The iPhones are still locked to AT&T’s network so unless you are going to jailbreak it (and I guess that is the assumption), you are still on AT&T.
Apple did this last year as well. AT&T, for their part, are again selling refurbished iPhones at a $50 discount.
Update: An Apple Store Employee has confirmed the new plan and will be selling iPhones from the store this way at one unit per person per day. According to the employee we talked to, they would still be officially tied to AT&T, “but everyone knows it takes 2 minutes to unlock them”.
Update 2: Gizmodo got the official doc, above: