As far as I know SoftBank's rules are restrictive about not allowing 3rd party devices on their networks and/or charging ridiculous rates if you use a 3rd party device. They claim to use IMEI information to detect non-SoftBank devices from using their data network. Anecdotal reports seem to indicate they "look the other way" most of the time - but this can change at any time. Personally, I avoid SoftBank data for exactly this reason: too much trouble, too much risk.

For a rented SIM, the worst that could happen is that they claim you were online 24/7 for the whole time you had the SIM and charge you for every second of usage at e.g. 10x the normal rate for using a 3rd party device.

I would recommend the new b-mobileSIM, which is a prepaid 2980 yen SIM card for one month of unlimited data usage: http://www.bmobile.ne.jp/sim/detail.html

I haven't tried this myself, but I have tried the older b-mobile 3G 150 hours modem, and used its SIM card in the N900 successfully (see here for the magic command needed to get the n900 to recognize the SIM - reboot after entering the command).