A few years ago, I co-developed the software and hardware setup of a system that tracked walkers in large scale events with the use of RFID technology. We integrated social media (Facebook) to allow for viral marketing as well as demographic/event tracking of participants in real time. It was a really fun project that allowed us to work on some very challenging issues.
You can read more about a press article that describes the details here: http://www.rfidjournal.com/articles/view?9837