From my understanding, in typical Japanese form, this cuntry has really fucked this up making it painfully expensive (and almost impossible) for visitors.
I have a colleague cumming from HK w/ an unlocked (of course!) phone. Last I checked, you had to have a visa/gaijin card to get a prepaid sim card. Is that still the case? I've found info on the internets very lacking. The idea is to avoid those 10 year old phones they rent out at Narita.