I think AT&T charges something like $1.30 for international roaming per minute...where they get you is on data usage, we went to Tahiti a couple of years back, and we used a bunch of MMS messages to send pics to family and holy crap, we got hit with like $100 over our normal bill (they charge like $2/message for MMS). We were also using it for tethering so we could skype and show my dad the view from our room and like 5min. of data usage was some stupid amount.
At least it worked though, before 3G service was around, we went to Italy and i had a blackberry at the time and it didn't work at all over there...it never even tried to connect to a roaming signal, which sucked (but thankfully we had someone with us that was from Italy and had a phone we could use that didn't cost them anything...
I believe they have a plan though if you go out of country often, it sent me a text message asking if i wanted to join the world hopper plan or something like that...