Schneier's blog tips an article about research into geolocation that can track down a computer's location from its IP address to within 690 meters on average without voluntary disclosure from the target. Quoting: "The first stage measures the time it takes to send a data packet to the target and converts it into a distance – a common geolocation technique that narrows the target's possible location to a radius of around 200 kilometers. Wang and colleagues then send data packets to the known Google Maps landmark servers in this large area to find which routers they pass through. When a landmark machine and the target computer have shared a router, the researchers can compare how long a packet takes to reach each machine from the router; converted into an estimate of distance, this time difference narrows the search down further. 'We shrink the size of the area where the target potentially is,' explains Wang. Finally, they repeat the landmark search at this more fine-grained level: comparing delay times once more, they establish which landmark server is closest to the target."