As an earlier thread states, I spent time in Japan earlier and the group liked it so much, we'll be doing it again in Summer 2012.
One thing we found was that sharing one cell phone between the group was hard. The budget phone we borrowed while we were too was also not too friendly on the language side.
I have a DoCoMo N504i that I found at a shop here. It's an 800mhz phone from 2002, has the basic functions and you could switch the language to english. The battery also seems to still behave as well.
Thing is, we don't know how the phone registration system works there. Are we able to go to a dealer, get it activated, then slap a temporary phone number on it for two weeks for a small amount of money or are phones permanently bound to the registree (kind of like some license plates are registered to one VIN) and can only be used under only one contract?