To Kumainjapan-san
Sorry I can't reply private message (don't know how yet) so I post it here because it is not off-topic.
If you read all the posts on this thread, all the confirmation you need is there. Just insert the softbank sim and you can make calls, sms without problems.
The N900 find open information stored on the sim and automatically set up an APN called Softbank Mobile Internet, with crazily expensive rate not covered by any of softbank discount plan. Before using internet, go to connection setting and edit this connection using instruction on my post or similar, and you will be able to enjoy softbank unlimited data plan.
For S!Mail, which is MMS, I still cannot make it work on my N900 eventhough many have claimed to have made it using fMMS app.
If you make a google search on Softbank setting, you will see the general advice is not to ask softbank directly, not to show your N900 to them, keep your profile low in case you have trouble with over charged. This is, sadly, the truth. Softbank staff can even put a ban on your phone imei, which is the worst, because you will not be able to put any japanese sim in to use at all. So don't ask them about phone they don't sell. You have many kind and ready-to-help friends here on this forum and on internet.
"Softbank will find out and block your phone" kind of reply from japanese staff is, to me, empty threat. Don't worry. They won't be able to. As far as I can guess: softbank asks phone makers to put proper useragents to those phone to be use on softbank, lock down those function softbank don't want to be used on their network, like PC direct, internet sharing, internet modem...(That is maybe why it takes so long, if ever, for a nokia phone to have softbank firmware update)
So if you find our proper useragent for the relevant phone, you can use any foreign phone on softbank network without problems.
The only warning applicable is, even in unlimited package plan, softbank can ban your internet access if your internet data is over 300'000 yen/month (I haven't tried, so can't confirm)