I lived in Japan for three years and while I was there purchased both OS X 10.3 and 10.4 without any problems. I lived in Kyoto and was able to get 10.3 from a tiny computer store in a small alley of shops. Although the disc is multilingual, they even had a copy from the United States for customers who wanted the English box and documentation.
When 10.4 came out, Sofmap (an electronics retailer that you will almost certainly find in any reasonably-sized town or city) even had a launch event. Of course, since Japan is in a much earlier time zone than the US, the launch line time was a much more comfortable 18:00! The line was also formed in an orderly fashion at a certain counter inside the shop, so I got my copy before most people in the West, and I didn't have to stand around outside an Apple Store at midnight to do it!
All in all, I don't think you'll have any problems. The caveat is that I speak Japanese, but really in the case of Sofmap you should be able to walk in on the launch day and find advertisements or standees in-store for the new software. Even if your lack of linguistic ability causes problems, Amazon Japan has an English option on their site:
http://www.amazon.co.jp/In-English/b/ref=topnav_switchLang_gw?ie=UTF8&node=1094656
It doesn't cover every single word on there but has the basic interface in English and should do you fine for ordering OS X 10.6 when it comes out. Domestic delivery is free over 1500 yen, so you won't even pay shipping. You can even opt to pay cash-on-delivery, where you just pay the delivery guy cash when he gets to your place, which is an option I found convenient when I didn't have a credit card.
Oh, and you'll be able to buy very cool Japan-only Mac, iPod and iPhone accessories
Incidentally, where is it you're going?
