I picked up a mobile wifi device from Narita this morning from SoftBank (B1 of terminal 2).

Prebooked onlineand the pickup process was about 3 minutes. They've got a special on at the moment - 1290 yen/day, but usage is limited to 1G (after which the speed is throttled).

Working well so far (and as others have said, it's a lifesaver for getting around and staying in touch with the missus back home).