No, you don't need a passport if you are there for less than 90 days. I was there end of march beginning of april and they are fine. Had a layover in sri lanka on the way out which was fun, think i'll fly direct next time!
Some really great things to be seen out there, have to experience the arcade...and the toilets! hehe. I managed to get out of Tokyo and went to see Kyoto and Osaka, which was amazing. I didn't half feel out of place at times, being a tall white guy surrounded by japanese people. There are no westerners outside of Tokyo that I saw, and only in a couple of places in Tokyo.
Took so many pictures, had a wonder round a pachinko place, but then I had to leave, starts making your ears ring eventually. Really enjoyed it, and just to confirm what others have said, no injections etc needed.
To the chap a couple of posts up, I went up the Tokyo tower, though only to the first level-bit, went up just after the sun had set, so managed to get some awesome pictures of tokyo at night, but I agree it was expensive, but then so is everything else in Japan!
Have a good time!
EDIT: Woops, obviously you need a passport, silly typo on my part, I meant you don't need a visa. Obviously you need a passport to leave the country. Thanks for pointing that out, feel a little silly now
. From the Mori Tower, on a grey day...
especially if you speak Japanese.