Becky Stern over at Adafruit added a Charlie Card (transit pass, Boston style) to an iPhone. This is a pretty nice hack. Although, I’m not sure how many people want to pull out their smart phone at the turnstiles–maybe a lot. It certainly decreases one’s chances of forgetting their pass. Even if you dont want to add this to your smartphone, this technique could be used to meld an RFID card into any number of, potentially cheaper items.
Check out the RFID iPhone tutorial at Adafruit for the full details. By the way, the new Adafruit tutorial system absolutely rocks! Nice work!