Nokia will soon have devices that has 2-axis acceleration sensor built-in.
How about using it to increase the accuracy of GPS tracking? It should be quite easy to calculate vehicle position using old position and then applying accelerations to it to get new position until next GPS update.
That way accuracy of track maps could be improved. Also, acceleration caused by throttle/brake and steering could be logged. I have not read the specs trough for the G sensor interface but it's update rate is way better than 1 Hz.
In optimum setup you would place GPS BT dongle on your dash to get clear view of the satellites. Then you place the mobile near the center of gravity of the car (between drier and passenger seat usually) to get most accurate results.
Any comments? :)