ATT/Cingular is great. Hop off the plane turn on your phone and you are all set. All your friends and family can call you at your regular number. And all you need to do to call the US is dial a + (press and hold 0) then 1 (area code) and the number.
Even your voicemail will work and text messages.
Cingular authorizes your phone for international use with a simple phone call for free.
Traveled to 15 countries with Cingualar and it has worked from the pacific coast of mexico to dunes of Dubai to Amsterdam
If you intend on making alot of calls and talking for a long time (you should be on vacation here hint hint) then pay the $5.99 discount plan fee for the month. Unlike some features you can remove this discount plan without a penalty.