Last year TomTom announced their HD Traffic system
http://www.tomtom.com/news/category.php?ID=4&NID=389&Language=4 that is using GSM handset location information from Vodafone to determine traffic conditions on dutch highways. Every 3 minute TomTom gets raw, anonymous handset location data from Vodafone. This data is then processed to determine where on the high ways large concentrations of mobile phones are moving slow. This might mean that there is a traffic jam at that location.
Each TomTom with HD traffic will also be equiped with a GSM sim and this enhanced traffic information is send via GPRS to these devices.
Some people are questioning the validity of their claims that this give better traffic details that current methodes. What is a car has broken down? will it trigger a traffic jam notification? How about paralell roads that are jammed? Or taxi's waiting at the trainstation for a pick up?