Maybe not as big an issue as thought -- it seems you'd start with the paper then move on to plastic. From the article:
T officials concluded that the students had, in fact, found a way to break the paper Charlie card system, but had only found theoretical methods for breaking the plastic Charlie card, an RFID smart card that can have T fares electronic added to it.
Slashdot Mirror
Maybe not as big an issue as thought -- it seems you'd start with the paper then move on to plastic. From the article:
T officials concluded that the students had, in fact, found a way to break the paper Charlie card system, but had only found theoretical methods for breaking the plastic Charlie card, an RFID smart card that can have T fares electronic added to it.