Rental SIM cards are available from Softbank, but they charge 2JPY per packet. Overpriced. I don't know if docomo rents SIM cards for their fOma network, you may want to check that out.

If you know someone in Japan with a cellphone, you can get a bmobileSIM. The 6 month and 1 year models are available in stores, while there's a 1 month version you can order online and have shipped to a Japanese address.

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

A friend and I used the 1 month SIM when I went over for 2 weeks last August. Had to go through Rinkya, a proxy-bid service, and paid an extra 2/3 the cost but it was worth it compared to the 6 month version. Had a friend in Japan activate it, worked like a charm in my N900.

I used Skype for all my voice calls while I was there and it worked quite well.