Most of the electronics today for a computer will work fine. Japan uses 110v 50hz as opposed to 60hz in the US, but today's adapters will easily handle the change. The only issue is that some do not use polarized plugs or grounding plugs, so you'll need to pick up a 3-prong to 2-prong converter just to be on the safe side. Cell phones will NOT work there (different system completely) so you'll either need a calling card or a phone rental while in country which can be picked up at the airport when you arrive. Oh, and Japanese ATMs will not accept on average accept your ATM/credit card, but as of July 11th, the 7-11 chain in Japan will have ATM machines that will. You can also go to the Japanese Post Office where you'll find one that'll work with the US systems (Cirrus/Plus/MasterCard/Visa).

Here's a good link for travel info:

http://www.japan-guide.com/