Most likely the AT&T site will let you set your order up and pay the subsidized cost of the phone (i.e. 199 or 299 depending on if you want 16 or 32 GB) it will then start to process it and about an hour or so later you will get an email that says processing error and to please call AT&T.
But you may have dodged the down payment with AT&T. You could call AT&T Customer service or roll into a AT&T corp store and find out. This way you can get your phone at the same time