If your mobile provider is ATT, head on over to http://att.com/deviceunlock and request to unlock your phone. This process takes 3-5 days, so do it before you leave if you’re heading international. If successful, you’ll be able to buy any local (compatible) sim card, and have a much cheaper talk/text/data plan that paying the Int’l roaming rates.
You’ll need you phone IMEI number, which you can grab without powering off your device by dialing
#*06# (UPDATE: *#06# is correct) and pressing SEND on your phone. TIP : TRICK: Do this for all your devices, even your old unused devices. They are great hand-me-downs, give-a-ways, and backup phones, when you break your screen 🙂 Why not do this now, even if you don’t have a trip coming up. Best to have it unlocked so if/when you need it, it’ll be available for you.