Don't quote me on this, but I believe it will work if you have a sim card that you can use in Japan, such as a sim card from a Softbank prepaid phone.

I believe this will work because:
1. The N900 supports the 2100 MHz 3G band, which should mean the phone will work in Japan.
2. There are reports of other 2100 MHz 3G phones working in Japan with a Softbank prepaid sim card. See for example here: http://talk.maemo.org/showpost.php?p...8&postcount=51

This is for the phone call functionality. As for messaging functionality, you may need to configure the phone with information that e.g. Softbank will not want to provide. But I think in 2010 the law will change so carriers must provide all configuration information to allow other phones to work on their network.

This is only for Softbank prepaid sim cards, which is a configuration that I have heard will work. I don't know anything about Softbank contract plans, or other carriers. I think I heard that non-standard sim-like technology has been/is being used in many Japanese handsets.