I am pretty sure that if you have an ATT sim card the phone does the activation on its own. The reason I say this is because I bought an unsubsidized iPhone 3G and I stuck my ATT 3G sim card in to the iPhone (with no iPhone plan, just a normal plan) and the phone activated itself and works great. Indeed I had to install a fake APN profile so I didn't get a crazy bill but I have been using it for a while. I don't think they do anything at the ATT store besides put a working sim card with an account on it in your phone. It's not like they plug the phone in to the computer and "activate" it. I know if you get your phone replaced at the apple store they just swap out sim cards. They don't even check your plan or "activate" it. In fact once they put your sim card they give you the phone. As long as you have an ATT sim card you are good. Im not sure where I am going with this.
As far as your problem I have no clue. Maybe it's your account (ATT's problem)? Maybe it just needs a restore. Or maybe you have no service. Or maybe it just didn't activate correctly (have you tried taking the sim card out and putting it back in and letting the phone activate again). Who knows?