For a long time I had no real japanese input on my N900 till i found a japanese blogentry (http://d.hatena.ne.jp/kansamjp/20100114/1263448115).
As not everyone understands enough japanese i will try to give a summed up translation:
1 install the packet "maemocjk-him-scim-anthy" from the extras-devel repository.
2 reboot the N900 and then activate the virtual keyboard in the textinput settings.
3 to use scim as default input method you must add one line at the beginning of the file "/etc/osso-af-init/af-defines.sh":
4 If you want also mozc input method install the scim-mozc packet.
5 then reboot for one last time.
Now you can use scim in most input fields (unfortunately not in Opera) by pressing Ctrl + Space to activate scim. Then the scim toolbar should be visible where you can also switch to mozc input method by clicking on "anthy". Unfortunately scim breaks the normal virtual keyboard which i think is no big deal on the N900.
Posted on Sun, 30 Jan 2011