Yes AT&T offers international roaming in Japan and 200 other countries. Here is there international features page
International calling rates and packages | Wireless from AT&T, formerly Cingular. However, in order to use international features you need to call them and have it set up and you aren't assured of it being activated unless you have had your account for 90 days. You also need a phone that operates on the UMTS 2100 frequency, of which there aren't very many available in the US. I think there are only 4 from AT&T; the AT&T/HTC Tilt, Samsung Blackjack II, Iphone 3G, and Sony Ericsson Z750a. When I was looking I couldn't find any other US carrier that had phones compatible with Japan but I last checked in April when I bought my Z750.