Just my 2 cents of what I know about Japanese Cellphones:

1) Although GSM is used in Japan, it is not compatible to other GSM services from other parts of the world. Therefore a regular GSM phone from countries outside of Japan will not work in Japan.

2) If you want to use your GSM cellphone in Japan, it must be the newer 3G capable (UMTS/WCDMA for Docomo and J-Phone and CDMA2000 1xEV-DO for KDDI/AU) ones.

3) You must also activate the international roaming service of cellphone provider and hope they have a "relationship" with the big Japanese cellphone services. Here's T-Mobile's Japan partnership

4) Pre-Paid cellphones were recently outlawed in Japan due to the "hello, it's me" phone scam and heavy use by the Yakuza in coordinating criminal activity. Apparently it was too anonymous and disposable making tracing the owner impossible. The new law also tightened up the ID requirements for all those wanting to activate service.

5) You can rent cellphones at the airport and most hotels. I've done this already just for me a my friend to be able to coordinate where we are at. Incoming calls are free. You are only charged for outgoing.

Okay, end of my 2 cents...I think the operator now wants me to drop a couple more dimes in the slot...