pourin' one out for all my netscape NOW! homes.

Ok, some of these guys have the right idea but they might not know about recent japanese law changes.

To combat the use of illegal phones, you CANNOT purchase a phone, preaid or contract, unless you have a gaijin registration card, which you need to apply for at the ward office of wherever you're staying. You need this card to buy a phone.

To rent a phone, DO NOT rent at the airport, it's expensive.

Myjapanphone is a good choice, as is
http://www.pupuru.com/en/index_en.html
Pupuru for cheap phones.