Well maemo mapper pulls from google maps, so Japan is covered...as long as you can read Kanji. Actually, I rarely suggest this, but you might be best served by starting a new thread to ask this question, with a nice descriptive title like "maps of Japan for Maemo Mapper?"

Now, slightly off the topic of Nokia Internet Tablets, I just wanted to give you a suggestion for a good paper map book to use in Japan: http://www.mapple.co.jp/publ/dekkaji.html
Don't worry too much about buying it before you get to Japan. Just stop by a book store first thing, and look for that style of cover. Basically what's good about it is that they have station names and area names in Romaji as well as Kanji. Other random thought: Here's a really good site for figuring out what train to take to get to what station: http://www.hyperdia.com/cgi-english/hyperWeb.cgi

Ah, random other thought: I think if you pick a big enough brand name of GPS (not an N810) you might be able to purchase a Japanese map pack for it.

-John