I use Japanese intensely and have Japanese input on the N900, however I do it like I do Japanese input on my notebook, not bothering to install a Japanese IME, instead write Japanese in GNU Emacs, than copy/paste it wherever else.

This works very good, I've even compiled Prime, a predictive input system for Japanese, which is great for a handheld device, you just enter the first one or two characters and get predictive proposals for the word you wanna enter.

I'll be posting info on how to setup GNU Emacs with Prime on the N900 at some later point in time, so will drop a note here if you're interested.