Here's what Boy Genius said:
According to some newly surfaced AT&T account codes ... we can infer that the iPhone will be available to Go Phone Pay As You Go subscribers, Go Phone Pick Your Plan subscribers ... and postpaid users.
This is good news for AT&T customers whose accounts are not ready for upgrades. It's also good news for non-AT&T customers who perhaps were on the fence about switching from their current carrier to AT&T.
For people who want the iPhone for its music and other media features, but not necessarily its telephony, this provides them with the justification they need. They'll be able to buy the iPhone and use the phone features at will, all while being able to enjoy the device for its other attributes.
One negative here, however, is that the prepaid accounts may limit the iPhone's true potential as a telephone. There's no word if features such as the visual voice mail would be available to prepaid customers.