As I always have my phone (HTC HD2) in the car (mounted and visible with a nice big display) for Sat Nav etc ... it made sense to use it for lap timing too.
I've started using RaceChrono:-
http://www.racechrono.com/
With one of these:-
http://www.qstarz.com/Products/GPS%20Products/BT-Q818XT-F.htm
The comination works beautifully.
Very impressed and all for less than 50 quid (assuming you have a suitable phone already!).
P.