I use a gopro hd hero and a cruise cam mount. The camera shoots decent quality for its size, and the camera mount mounts easily to the passenger head rest. Both together will cost about $360. So it's a pretty inexpensive way to go and the results are pretty good.
I recently got a canon T2i which shoots hd video also, so I may try that next time.
///M3 E90 '09 Le Mans Blue | DCT | Silver Novillo | Tech | Prem | 19"s | Prem Sound | iPod
StopTech BBK | Volk VR.G2 | H&R Sport Springs