Take a look at

http://www.bmobile.ne.jp/payg_sim/

Bmobile's political connections have prevailed again and it is offering a prepaid SIM that includes

-- 60 minutes of calls (domestic or international)
-- 3G of data
-- almost certainly on the DoCoMo network, best in Japan

The problems are

-- Only good for 7 days
-- Costs a ridiculous Y9,980 = US$93!
-- Cannot be refilled or extra data/minutes added
-- Has a fair amount of bureaucracy, including emailing in passport photo copies not once but twice

Still maybe in 6-12 months, there will be competition and prepaid for visitors will finally return to Japan.