I don't know the correct answer to this, but I'm going to say no. However, they will probably require, at the very least, a 1-year contract. When I sold cell phones, the carriers generally needed some kind of contract even if you brought your own phone.
My best advice would be to just call up an AT&T store and ask. Just ask 'I have my own AT&T phone, if I activate a new line of service, do I have to sign a contract?'
If yes, I would probably just shell out for a new iPhone and sell the old one on ebay or something.