Hi Shadow
I notice that nobody has suggested renting a phone from Mobal at Narita airport (I assume you're going through there?).
They currently have a special 'free rental' offer (at least they did when I last looked). So you could have a really cheap solution for your trip.
Here is a link to their website - have a look
Japanese Cell Phone Rental from Narita Airport, Tokyo
I used them when I went to Tokyo and I would recommend them.
