I bought a new Nokia 7600 when I was in Helsinki. When I landed in Tokyo, my Docomo phone was not working. (I think I missed a bill payment.) But my Nokia 7600 roamed on the J-Phone WCDMA network without a hitch.
Probably only people who have travelled to Japan from GSM countries will understand how cool it is to be able to use your GSM SIM card in a phone in Japan where we don't have a GSM network.