i was in japan again last month. i borrowed a vodaphone/softbank SIM from a friend and it worked perfectly in the n900 in 3g mode (no GSM available).

wireless worked fine when there was wireless available, which was almost never.

here is the old thread we were using for this topic:
http://talk.maemo.org/showthread.php?t=31680
there is a bit more info in there if you manage to wade through all seven pages