I bought 2 AT&T GoPhones mainly to keep in contact with other family members on a trip to California this month.
Cheapest handsets are $30 + $25 credit on each one + $1 a day giving free calling AT&T to AT&T; worked fine esp as roaming calls on O2 appear to be £1.99 a minute!
SIM cards do not work back in UK on an unlocked handset but I didn't call AT&T to ask about roaming and I forgot to do it in the store.
Interestingly, AT&T say that the iPad comes with a SIM card and you can activate and use data with a non-US credit card - I didn't try it though as I had free or cheap wifi in all the hotels used.