What you should do is return the phone and cancel the iPhone upgrade contract. You are well with the 30 days and will have no problem doing this. This would, of course, disallow you from giving the phone away, but you can't give 3G iPhones away anyway. ATT ties every phone to a contract. Without giving them the phone(and being within 30 days of activation), you can't cancel your contract without an ETF. And similarly, unless you buy an iPhone directly from ATT you can't get service on it, because they assume it was obtained unlawfully.
This is likely to change as more and more people cancel their contracts early, selling their phones; right now however, unless purchases accompany contracts you can't get a contract(or probably have the phone on a month to month basis).
Take the phone back, get a refund(!), avoid paying an ETF, then switch carriers/phones/whatever. I don't see you having another option. And if you did, it would be a lot of hassle for a little reward.
Then again, you can sell the phone to some twit for a lot of money, conning them into thinking they can get a contract. That might be unethical though.