This is kind of an old project - but I don't think it has been blogged yet. The "T" passes for the subway in Boston are great - but they are an extra card that you need to carry around. They work with RFID so the essential parts are an antenna and a small chip.
If you soak the card in some turpentine you can with-care separate the plastic bits from the antenna and chip. I slapped mine on to the back of my phone. The first time I gave it an extra loop so it could fit underneath a round Loud Bicycle sticker. The next time I just put it underneath some clear packing tape which made it function a little better.
If you are trying this for the first time - please make sure to use a Charlie Card which doesn't have much cash on it just in case you toast it.