You can now rent a SoftBank (ex Voda) SIM for 105¥/day (about $1/day) with free incoming calls but rather spendy outgoing calls.
http://www.softbank-rental.jp/inbound/eng/phones/
Since I now have an O9 SIM I plan to test it in Japan as I pass through on my way to Singapore on the 26th. I'll post the results.