Use SIM card with iPhone
I bought a used, unlocked Apple iPhone 3G from a guy on Craigslist. I also bought a Nokia 2610 GoPhone from an AT&T store. It came with free minutes almost totaling the cost of the phone. Then I took the SIM card out of the Nokia and put it in the iPhone (I didn't mention that I was going to do this when I bought the Nokia).
I don't have an AT&T contract for the iPhone, but I can use voice and SMS pay-as-you-go as with any GoPhone (I don't use that very much, since I also have VOIP as Google Voice or Skype), and also I can use the wi-fi connection wherever that is available, instead of AT&T's poor 3G network, which is unavailable to GoPhones.