By now the secret's out - they speak Japanese in Japan. If you have little or no proficiency in Japanese upon your arrival, don't worry; most JETs are in the same boat (or maybe the same plane?) as you. Japanese language ability is not a job requirement, and you can get by without it, especially in a big city like Nagoya. But you will greatly enhance the quality of your life here if you attain at least basic proficiency in Japanese. It is strongly recommended that you at least make an attempt to learn the language while in Nagoya.
If you are starting from scratch, and want to get a head start on Japanese before your arrival, it's probably best to begin with simple politeness phrases, and with the 46 hiragana and 46 katakana symbols that make-up Japan's alphabet(s).
You might also consider checking out any of the following websites. They offer free online Japanese language instruction:
Japanese Online
Free beginning Japanese lessons online at http://www.japanese-online.com/
Nihongo Web
Another similar site with free online lessons. http://www.nihongoweb.com/
Sabontenweb
Japanese language learning tools on the web. http://www.sabotenweb.com/bookmarks/language.html
Jim Breen's Japanese Page
An extensive index of links for learning Japanese language and culture, and for Japan-related computing questions
http://www.csse.monash.edu.au/~jwb/japanese.html
During August, you will be treated to a free, short, crash-course in Japanese, tailored to your level, at Nagoya University (see The August Files). After your crash-course, in your in-service training seminars, you will be briefed on ways to continue studying Japanese while in Nagoya!