Both NTT Docomo and Softbank operate on the 2100 Mhz W-CDMA band which is compatible with most of the global 3G network providers around the world so now, most 3G hardware available on the open market is technically able to work in Japan. I say technically because there are still quite a few safeguards put in place by Japanese carriers to prevent both exporting and importing.
Nice short guide to the limitations on mobile in Japan.
