FRS, I was in discussions for making something similar for a project with my university. I wanted to develop a Windows CE program to record your route around the track, so when the lap is done you can compare your lines along with many other laps all overlayed onto a map of the circuit. As well as recording instantaneous speeds all the way round. Possible addons could include accelerometers for measuring g-forces around the track also.

Basically a program to datalog readings that we already take (e.d. Datron) but to overlay them onto a map.

In the end I decided that GPS couldn't pinpoint your location accurately enough. (I forget the exact accuracey, I think it is 2m) Either way it wouldn't be great for examining your lines!