This feature would be very cool. We are using IGuidance as our in-car interface and Google Maps to track vechiles from the web.
The google Maps API has spawned so many cool projects. One example:
An app that monitors gas prices and plots them on a map. Based on price the user can work out the cheapest place to fill up.
If we could pull this info already available on the webserver and import it into IGuidiance think of the hacks that could be made.