I've lived in Japan for several years in the past (in Tokyo), and was vegetarian while there. I have been vegan for the past year and don't intend on changing while in Japan, and I'm sorry, but for the person suggesting to essentially 'just shut your mouth and eat the pork' if its being served at an event, that is just ludicrous for most vegetarians and not very open-minded advice. Anyways. I've just been placed in Tokushima-Ken, Shikoku, so if there are any veg heads headed that way (or already there!) I'd love to hear from you!
I had no problem being veg while in Tokyo, but to be honest this was mainly because I mainly ate out at veggie/indian/italian restaurants and was spoiled with international supermarkets. I know that most local/Japanese restaurants will be generally quite difficult to eat at, but from what I've read, in smaller towns several restaurants are happy to accomodate veggies. I know for sure that the most difficult scenarios will be school lunches (almost never veg-friendly, and as several people have mentioned it is probably best to tell your school/BOE etc that you will be bringing your own lunch) and enkais and eating out in general. I think the best advice, I would imagine, would be to politely tell people in advance about your dietary lifestyle, and to always assume there may not be much to eat at these events, and hopefully people will be pleasantly surprised/accomodated! And of course, for those of you who don't speak or read very much Japanese, memorizing certain kanji and phrases will be vital.
And its true, you will find animal products where you least expect it. I remember buying french mustard once at the grocery store only to notice it smelled a little strange once opened. My roommate checked out the label and confirmed there was pork product in it. (?!?! in mustard!). Also, there really is dashi (fish stock) in virtually every sauce in restaurants, so just be aware of this.
On a more positive note, I know that cooking vegan in Japan will be totally doable. I can't speak about rural Japan 100%, but there are several brands of Japanese soymilk (which in my opinion are tastier than many of the Western versions, my favorite is the brand that comes in a green carton with a sun cartoon on it) which are available at most supermarkets. And of course, the ubiquitous tofu is widely available, and though often quite expensive, fruit and veg are widely available and of high quality. There are also brands of vegan Japanese curry, and most store-bought yakisoba (fried noodle) packs that come with a powdery flavoring are also safe, last time I checked!
Other people have linked to websites that deliver natural foods. I have not used these sites personally, but have heard they're quite reliable, and I'd imagine super useful for ordering products not widely available in most Japanese supermarkets.
Several people have posted links to the 'Kansha" book, and I also have a cookbook called The Asian Vegan Kitchen, which was written by an Indian woman living in Tokyo, which includes a substantial Japanese recipe section, I would highly recommend it! (http://www.amazon.com/The-Asian-Vegan-K ... 477003069X
Additionally, I found a former JET's youtube channel which includes about 10 videos on being veg in Japan, cooking, products to stock up on, etc:http://www.youtube.com/watch?v=SxkIJ8tM ... el&list=UL
Lastly, for anyone planning on spending some time in Tokyo or Osaka or placed nearby, it is useful to check out the respective cities' Vegan Meet-up Groups' websites. They are a great way of discovering places that might not have much of an online presence in English, or that might be harder to find. Also, veggie food is definitely more popular in major cities in Japan than ever before, with some fast food outlets serving veggie burgers. (Check out Freshness Burger's 'tofu hamburg'!). One last word of caution, many seemingly 'veggie' burgers/hot dogs contain meat, so always double check before ordering!
I could go on and on on the topic so if anybody wants to PM me with any specific questions, please do! (Again, i've never lived in rural Japan but have spent several years in Tokyo and can comment on veggie restaurants there as well as on the standpoint of using Japanese ingredients to cook vegan food.)