You could roam on DoCoMo instead? Or Vodafone could do a deal with Softbank (Vodafone Japan is now classed as a "company that ceases to exist" so it's not like Vodafone would be dealing with Vodafone.)

Maybe it's because Japan has zero GSM support... I can't find the list that the register is referring to, but on the page on Vodafone's site about the passport service the list only seems to contain GSM countries and one 3G country.. Japan.

Anyhow, I have o2's unlimited data plan thing (200MB) and never seem to get charged for roaming with it. Which is nice. :)

/me is sitting only a few foot away from a NTT cell site.