Options for using an ATT iPhone on a prepaid plan?

[rgG]

My daughter has been on H2O wireless for a couple of years, with a locked AT&T iPhone 4.
No problems.
She will be switching to Verizon, on her grandmother's plan, for the coverage and a new phone, but the H2O service has been fine.

Note, if you go with H20, you can buy your refill PINs from CallingMart, and usually save 5%, with an email code that they send out about once a month.