An even better option is to go prepaid with H2O Wireless. It's way cheaper than ATT

It's super easy and super cheap, just $.05/min and $.05/sms or a $30 unlimited talk and SMS plan. They have other plans too with data and the like.

H2O works with all ATT phones.

It's almost free to try, just go onto Ebay and you can buy a SIM for $1. Log on to their website to activate.

Pop it in, and you're going!

I use this when I'm back in the states with my iPhones and they're awesome, highly recommended.