Nike and Apple combined forces to create an iPod nano that would work with running shoes, allowing you to listen to music and also keep track of your progress, e.g., time, distance, pace and calories. An RFID sensor fits into a pocket in a special shoe and relays information to a small receiver you connect to the iPod. The Nike+iPod Sport Kit is a great idea.
The problem I’m reading about has to do with RFID. It broadcasts your encrypted progress information which can be intercepted, according to researchers at the University of Washington. It also broadcasts a unique identifier. With relatively inexpensive equipment, others can read this identifier, thereby being able to track that person. The full report is available at the University of Washington Computer Science & Engineering website.