As mentioned in previous posts, you cannot easily buy a prepaid phone or SIM in Japan. Prepaid phones are generally available only to residents, though there can be exceptions.

You can rent a SIM from SoftBank for ¥105 per day, plus usage. Incoming calls are free. Outgoing calls are ¥105/minute. Messaging and data charges are additional.

http://www.softbank-rental.jp/en/phones/sim3g.php

You might be able to find deals on rental phones. Passengers on ANA can reserve a phone with no daily rental fee, just usage charges. I think I've seen similar special deals from some of the mobile rental outlets in the arrival lobby at Narita.

I still find the usage rates for prepaid and rental phones in Japan to be very steep compared to other countries. I carry a prepaid phone primarily so that others in Japan can reach me, but for making calls I'll generally use a land line or pay phone for calls within Japan, and Skype or MagicJack for calls outside of Japan.

I've heard that Verizon CDMA phones may now work in Japan now through a roaming agreement with KDDI, but coverage may be spotty. I believe that Verizon's roaming rate may be over $2/minute.