Nope, unfortunately. AT&T's pay as you go service is not supported on the iPhone. There is a MVNO on AT&Ts network called H20 wireless which specializes in pay as you go and month to month, but you're not going to get 3G data. You're also going to want to check on the wireless coverage in the area you're going to be as GSM coverage can be rather spotty with a lot of places just getting 3G service late last year.
I will be in NYC so AT&T coverage won't be a problem. Are you saying that people in NYC have no option but to stick with AT&T if they want to use a GSM iPhone on 3G, even after expiry of their contract?
So much for competition.