Right, the N900 will probably work with Japanese carriers that use 2100 MHz. The question is the Band IX, that apparently is only used by the E-Mobile carrier here in Japan and nowhere else in the world. (See http://en.wikipedia.org/wiki/UMTS_frequency_bands) Still I guess there's hope that Nokia may support this band if they bring out a version targeted at Japan.

The good thing about E-Mobile, and the reason I was hoping I could use an E-Mobile sim card in the N900 directly, is that it is specifically designed for data transfer and PC tethering with flat-rate unlimited usage; they even sell a phone specifically designed to tether to a PC, including explicit instructions in the manual how to hook the phone to a PC for PC-based data transfers. In contrast to this, Softbank's 3G flat-rate data plan is supposed to be used on the iPhone only and Softbank apparently may hit you with a huge bill if you tether to your PC because it's not supported, and Softbank goes out of its way to try to prevent tethering at a technical level. So I imagine Softbank would frown on using their sim card and their flat-rate data plan with an unapproved and open device like the N900.

The prepaid ones are easier to get, but they have no data plan, as I mentioned, which is why I'm looking at a separate E-Mobile data plan.