Your problem is that you have an iPhone. ATT has a 100 meg prepaid option, I've heard rumors that they are not letting the iPhone on it because too many people were using jailbroken ATT iPhones on it. ATT actually has a prepaid plan for the 2g iPhone, but is booting 3g users from it.
You might want to jump into this thread over at Howard's Forum (in the iPhone forum) where a Canadian (who frequently visits the State's) is asking the same thing as you:
HowardForums: Your Mobile Phone Community & Resource - Is there a way to get data on a prepaid plan for an unlocked iPhone 3g 3.0?
Many people have got their devices running on the T-Mobile USA prepaid Sidekick option, but there is some minor system tricking going on:
http://www.modmyi.com/forums/t-mobil...-iphone-2.html