They're heading in that direction. With the Near Field Communication (http://en.wikipedia.org/wiki/Near_Field_Communication) standard. They want to embed a two-way RF device in cell phones (and other devices) that can emulate an RFID tag such as a Charlie Card, or be used to exchange money between two NFC devices.

With NFC technology, you could have various cards embedded in your cellphone (Nokia came out with an NFC cellphone in 2007).