Check the specifications on the Motorola or phonescoop website to make sure. As long as it's a quad-band 850/900/1800/1900 phone, it'll work in Japan. All you need outside of the USA is the 900 & 1800 bands. If I remember correctly, your phone is a quad-band. With that in mind, it'll work in Japan. So you're good to go if you're using Cingular's World Traveler Service over there.
Since I keep getting different answers, I was wondering if I could actually use my motorola V551 in japan. I am signed up with cingular and their world traveler service, but i've been told both my phone would work, and that it wouldn't work and i would have to get a phone from cellhire.com. i've called cingular and the last response i got is that my phone WOULD work in japan and I wouldn't have to rent a phone... but I was wondering if i could get some confirmation. If I would have to rent a phone, i'll rent a phone. I just really wish I could get a straight answer. thanks in advance.