Here's some notes about Japan. Doesn't appear to be much if any GSM there.
http://euc.jp/misc/cellphones.en.html#tech
But Telestial (common place for getting foreign SIM cards) seems to just say you can pop in their SIM and go... so I donno. I've seen other stuff online saying only HSPA works there (which is W-CDMA).
Worth a shot to try tho.
--Chris