Erin and Simon, over at Never Ending Voyage, have added a page on Japan to their growing list of resources for vegetarians on the road (other pages cover Hong Kong, and parts of South America).
I spent three-plus years living as a vegetarian in rural, remote Hokkaido. Then again, I lived with an awesome vegan cook at the time, so it was no great hardship for me. The reality for most vegetarians, however, and especially travellers, is that Japan is a surprisingly challenging destination for non-fish eaters, in particular, as this recent thread at Lonely Planet’s Thorn Tree attests.
In a pinch, you can always go Indian. Erin and Simon, however, provide some survival tips, such as useful words and phrases in Japanese, pictures of vegetarian-friendly dishes common in Japan (think tofu! And Mos Burger’s carrot and burdock-root “burger” on a rice cake bun), and a short list of restaurants — mostly in Kyoto — which the authors recommend.
All in all, a virtual tabehodai of useful information for vegetarian travellers to Japan.