If you are going as a tourist, a Japanese law that went into effect in late 2006 (I think) made it illegal for gaijin to actually own their cell phones. Some sort of anti-terrorism thing. I used to have a Dokomo account but was not able to re-activate the phone after that law was passed.
If you want to buy a Japanese capable cell phone.... let me know. I still have it <g>. You'd need to re-activate it.
In order to actually own a phone now, you need to have a resident alien card. Which means that you have to be living in Japan "long term".
Now I rent them when I am there since I am only there about 2 months at a time. The best deal that I have found is RentafoneJapaan. They are very easy to deal with and the price is right.
Rentafone Japan - Cell phone rental in Japan
best,
..................john