B-mobile mentioned before on this forum (if you can read/write enough Japanese to fill out the order form) has data only sim cards (and microsim for iphone 4 & ipad) for sale. The speed is capped at 300kbps but it should do most things you might want for mobile internet on the go.
Prior discussions on the regular sim card version:
http://www.flyertalk.com/forum/japan...apan-comm.html
Order:
Microsim version:
https://ec.bmobile.ne.jp/ecom/card_o...t.jsp?id=MU300
"regular" sim version:
https://ec.bmobile.ne.jp/ecom/card_o...rt.jsp?id=U300
Slower then wifi, but cheaper then any device, and works on the go, pretty much throughout Japan.
Various other blogs on the net talk about those items as well.
Unlike prepaid voice/phone plans, no Alien Registration etc is required for data only.
The order form is in Japanese, however when you call to activate the SIM (you have to use another mobile phone, since you can't use the phone the data sim is in, the activation can be done in English on their automatic phone system).