No you cannot. AT&T is not in the business of selling phones, they're in the business of selling phone plans.
Also, it seems Apple's agreements with carriers in the US prevents them from selling unlocked phones for at least some time after coming out with a new phone (at least that's my guess as to why they don't offer unlocked phones in the US for the first month or two).
If you can find one in an Apple store without reserving, you should be able to buy one as a device only without a plan. As far as I know, carrier stores won't sell you one without a plan, even if you buy it full price with no contract.
You can order one with AT&T full price with a plan, and then cancel the plan as you do not have a 2 year contract, but you might still be required to pay the first month + $36 activation fee, and the phone is not unlocked if you buy it that way.