This is a crazy concept but it sounds pretty sweet. Basically takes GPS data and transmits it to a game where you can join in on the racing.
Supposedly there is code to take care of overtaking (on screen, the real racer would overtake your virtual vehicle, rather than just pass through) and when a real racer and a virtual racer collide (they say the real car always drives away, but the virtual one is another story).
http://news.bbc.co.uk/2/hi/technology/7440658.stm