I have been experimenting with using the G1 GPS device to track my car while driving round a race track. It works pretty well if there is a good view of the sky and the phone can see plenty of satellites.
Here are some results from a track day at Silverstone (Grand Prix circuit). The green sections are where the car is accelerating and the red sections are braking zones. Green and red numbers show maximum and minimum speeds respectively in metres per second. The yellow numbers are peak cornering forces in g.
Copse corner, did an RX8 on the exit here :-)
The G1 samples the GPS device approximately once every second, and the accuracy is to the nearest couple of metres or so. It's good enough to see your lines although I ought to check against a professional datalogger just to validate the error.