Have you tried this in Japan?If you can't buy a pay as you go data microsim for iPad then buy a normal size mobile broadband sim and cut it down to microsim size.
Again, have you tried this in Japan?NTT docomo is selling data only prepaid SIM cards (3000 or 5000 yen i think) which are mini SIM cards. You can cut the SIM to microSIM size and insert it into the iPad.
Japan is not open. We are led to believe that we must use a SoftBank Micro SIMM in Japan. Looking forward to seeing your responses above.