I bought a Kindle 3 Wifi+3G while visiting the US last year. I've used it both in the US and Japan, and 3G is *free* in both countries. Even when there's no 3G access, you can still access the internet through EDGE coverage etc. I would highly recommend it if you live in Japan!
Buying books is free through 3G, too. You would have to pay for 3G transfer of personal docs, but that's what you've got free wifi for... ;-)