If you wouldn't mind reading the iPhone FAQ in the Guides, you would see that the current iPhone (GSM 2.5G) is not compatible with the networks in Japan because they have already phased out GSM 2G, they have been using 3G system for a couple of years and had no need to keep the 2G around. As a result, for the iPhone to be launched in Japan it will have to be GSM 3G (UTMS or FOMA).
TEG