Starting in 2013, the Paris Metro's new generation of Navigo transit cards may let you refill them by phone. Along with a new visual design, the cards will be moving to a new, phone-compatible system of near-field communication or NFC. This doesn't mean your phone will be able to double as a pass any time soon, but STIF, which manages the Metro, plans to let people refill cards by tapping them against an NFC-enabled phone. Although it's not in any current plans, this move could also open the door to using phones themselves as Metro passes.
Currently, Navigo cards are based on the NFC protocol B', which is not widely used. New cards will use the more common protocol B instead, making it possible to broaden their applications. The cards will be put into use on January 1, 2013, though STIF notes that older Navigo passes will still work for some time.