TomTom to track phones for real-time traffic info: "The system uses standard signalling data sent back and forth between mobile phones and cellular base-stations to pin-point a handset's location and speed, which are effectively the position and speed of the vehicle it's inside. Build a composite of all the data received this way from handsets located along a road and you have an indication of how congested - or not - that road is."

Interesting; I'd heard of this technique before, perhaps it was their trial...?