I posted this in another post before:
Japanese Cellular Phones FAQ
Information is probably outdated here, but should still have some relevency. Check to see if your phone support any of the listed transmission standard.
I can almost guarantee that GSM (With EDGE or GPRS), and TDMA phones will NOT work. CDMA MAY work, but I am not sure if there is any temp carrier setup in Japan to work with your service provider
I know for a fact that GSM will NOT work, I tried it, unless they changed it since October 2007, otherwise it will not work