I just came back from Japan a month ago and I had my N900 with me. Most mobile phone providers don't use sim cards but it doesn't really matter since as a foreigner you can't get a contract anyway and there are no prepaid phones except some hilariously expensive ones that you can get at the airport or in selected mobile phone shops (This page has good information also concerning rental: http://euc.jp/misc/cellphones.en.html).
The N900 works perfectly fine on the 3G networks but since I suppose you don't want to do data roaming this is only good for calling.
It was however quiet easy for me in a lot of locations (might've been lucky) to find free/open wifi connections and I never had any trouble connecting. This was good enough for me. Sorry I don't have better advice on how to be online whenever you want with your N900 (except data roaming of course).