When I interviewed Natsuno-san in 2001 for the book version of Smart Mobs, he made it clear that iMode had plans to make phones useful as payment devices. Textually reports that the phone as credit card (not just a prepay system) is happening:

NTT DoCoMo will shortly start “DCMX,” a credit payment service for its “Osaifu-Keitai (wallet-mobile phone)” compatible handset users, reports Tech-On. Users can do credit shopping without signature, just by holding the handset over dedicated readers at stores showing the “iD” logo sticker. Like standard credit cards, this is an after payment service that requires no cash pre-charge.