Japan is the third largest CouchSurfing community in East Asia behind China and South Korea. Japan's community is about 5000 people strong and growing. Over 40% of the members live in just five cities - Tokyo claims 27.8% and Osaka, Kyoto, Nagoya, and Yokohama each claim around 4%.
Currently the only complete city CS wiki page for Japan is the Tokyo page. Even if you are traveling in other parts of Japan, some of the information may be helpful, including tips and transportation websites.
Before posting a general question on Japan, check out both the Japan and Tokyo CSing wiki pages.