At long last the n900 is in my possession. I immediately tried using my 2 Japanese SIM cards: one voice-only from Softbank; one data-only from NTT Docomo (resold as B-Mobile 3G). The SIM cards are pictured in the attached image. Both SIM cards work when used in the original provider's devices.

The top SIM is Softbank and it works. The bottom SIM is Docomo and it doesn't work; on booting the n900 I get the error "SIM card registration failed". I tried reseatng the SIM card several times to no avail.

As can clearly be seen in the photo, the working top SIM appears physically different tham the non-working bottom one. Is that the reason for failure? I'm not so familiar with SIM technology. Or might a firmware update fix this? Or maybe Docomo has locked me out based on my non-Japanese IMEI?

Thanks for any advice in understanding and hopefully solving this problem.

Probably they are imei checking but, do you have the phone in dual mode, ie supporting 2g and 3g, some sims will only work on the 3g network or vice versa.
The different physical sims shouldn't matter, just different models, only difference is amount of memory on sim card for contacts etc.

The bmobile SIM is now working! Huge kudos to "tesuri" who has
shared this settings through his tweets.

As a root user, run the following command:

dbus-send --system --type=method_call --print-reply --dest=com.nokia.phone.net "/com/nokia/csd/gprs" org.freedesktop.DBus.Properties.Set string:com.nokia.csd.GPRS string:AttachMode variant:string:WhenAvailable

The settings will be saved permanently.

ok my reply is going to be completely irrelevant so please ignore me if you dislike off-topicers.

i never thought japanese people would use foreign phones except for the almighty iphone , i mean the japanese phones sold only within japan are so great, awesome design, technology wise, i think they had email and blogging services 10years before the rest of the world does, and most have ridiculously good cameras like the 12megapixals ones in the newest sharp series.

call me whatever you like lol, just expressing my amazement~

Excellent! I tried it and it works perfectly!

I'm on the prepaid 150 hours plan. I think they recently came out with an unlimited use plan for around 3000 yen/month. Presumably you now could use that sim also in the n900. Gonna give it a try when my 150 hours runs out.

Thanks a million for the tip!