I got a coupon code "Fuji" from the site Japan cell phone rental, Japan Blackberry rental, Japan cell Phone, Japanese cell phone-MyJapanPhone.com
They offer 50% discount on renting a b.berry (unlimited data!!). Their berry model is 8707, which can work in States, Europe, Far East, including Japan and Korea.
I'm planning to rent one for my trip, they're the one I found the most cost-effecient.
Any other recommendation?