I have an iPhone 3GS and an AT&T prepaid, about $32 a month after tax. It was kinda crazy, I had to purchase the prepaid SIM put it in my phone, make a call, then an AT&T guy did some typing and I put my original SIM back in my phone (so I got to keep my cell #) and viola - iPhone on the AT&T network, $32 a month! I can use my phone in Wifi spots and have limited (30mb) data plan when wifi isn't available. Highly recommend visiting an AT&T store and just let them know what you want to do. They were very helpful.