Greetings from a gaijin.
I am living in Japan and so happy toying around with my n900, Hongkong version.
I bought mine online in Japan. You can do it easily too from these sites:
- www.moumantai.biz (payment via Japanet Bank to deliver to your door)
- www.rakuten.co.jp (cash-on-delivery though you can transfer or use credit cards, my favorite because I dont have a credit card, and this site have English option to register membership).

For Japanese input, Kimitate has gathered a metapackage called Maemocjk-him-scim-anthy. I just installed this package using application manager. This work fine for me but I wish it could be better because I have to slide the keyboard in and out too often, making me worry for my N900 life-span!
I try making Japanese input as default by adding this line (learn from Internet teachers like you ofcause):
export GTK_IM_MODULE=scim
right at the last but one line in this file:
/etc/osso-af-init/af-defines.sh
This method work well too, but as a trade-off, I lose the symbol table. So whenever I need some symbols, I have to go to the file and delete the added line, reboot the phone!

For Softbank's Package Data Flat, I use this:
Conncetion name: Open Softbank
Connection type: Packet data (automatic anyway)
Accesspoint name: open.softbank.ne.jp
Username: opensoftbank
Password: ebMNuX1FIHg9d3DA
Prompt password at every login <box unchecked>
Use proxy <checked>
HTTP proxy <leave empty>
Port number: 8080
HTTPS proxy <leave empty>
Port number: 440 <not sure if neccesary>
FPT proxy <leave empty>
port number: 20
Installing Hide User Agent seems to be a must because with the above configuration, before installing Hide User Agent, I could not connect to Internet, but after pulling it in and press the hide user agent button, I have been connecting to Softbank easily for 4410Y/month.
For map, Ovi Map does not work well for me in Japan (only national roads and being located several prefectures away!!!). Google mobile map seems a little bit better, but not useful yet on my N900.
My last experience to share, and with which I so much hope someone else could enlighten me, is Softbank MMS.
I installed fMMS 1.2.5, have tried several useragent like N73, N95, Iphone 3GS (those version Softbank has) using gconftool-2, but the furthest I can have is receiving MMS headers, then "unable to retrieve message" or "Unknown name resolution" or the likes.
I gave a shout to the fMMS thread already. But the fMMS thread seems to be too big for Japan.
So please help if you can
Thanks.

PS. After PR1.3, Hide User Agent seems nolonger needed
fMMS 1.2.6 still not works for me, please help