Sorry, no solutions as of yet... I have temporarily given up, and bought a Sharp EM One when works on the E Mobile network. I also got a regular Softbank cellphone, (Toshiba 911T) and is patiently waiting for a solution. Right now, the Windows mobile situation is pretty bleak, with the X01HT being sold out, and its replacements being delayed for over 2 months. The solutions from wilcom is expensive and slow, being stuck on a 256 kbs PHS network, and expensive, with its unlimited data plan at over 12000 yen. At least the EM One is pretty capable, but large, and has no phone capabilities.
There is no debate that you can use the Treo 750 in Japan... I have roamed several times with it when it was with AT&T. I have put the softbank sim in, and it works fine, but the data portion did not work.