Bluetooth, a wireless link built into many cellphones, makes our movements trackable by anyone equipped with a PC and an appropriate receiver. Vassilis Kostakos at the University of Bath in the UK placed four Bluetooth receivers in the city's centre. Over four months, his team tracked 10,000 Bluetooth phones and was able to "capture and analyse people's encounters" in pubs, streets and shops.
For more serious scariness, check out the newest ThinkerNet blog over at Internet Evolution.
— Larry, Attack Monkey, Light Reading