Already a device of multiple disguises, from camera to music player and mini-TV, the cell phone’s next trick may be the disappearing wallet. Since more than a quarter of the people on the planet already carry around cell phones, why should they bring along credit and debit cards when a mobile device can make payments just as well? At the simplest level, all that’s needed is to embed phones with a short-range radio chip to beam credit card information to a terminal at a store register. This is already a reality in Japan, where NTT DoCoMo Inc. says 3 million cell phone subscribers use its Mobile Wallet service to buy things at 20,000 stores and vending machines.