I'm pretty sure using a credit card to activate a cell phone is pretty standard. I think it's so the carrier has one on file in case you decide to flub your payments. However, you can pay your bill however you want with AT&T. I usually do mine online via a checking account or credit card a few days before it's due.
If you can work an iPhone as a GoPhone, that's about the only way you can do it w/o a credit card.