The wireless running sensor that is a co-branded initiative of Nike and Apple has been shown by scientists at the University of Washington to be vulnerable to invasion of privacy. The device can be made into a tracking device and interrupted to erode personal privacy, according to researchers.
The scientists 'hooked their tracking device up to software that automatically plots a runner's position on an online map, and sends an alert by email or text message to an operator, when the target comes within range'.
It is unclear how far-reaching the extent of their claims is though, since the range of the signal is just 20 meters. Perhaps this is easily combined with other covert wearable surveillance equipment for maximum portability?
At any rate, the more details of their work can be found here and their publication here.