3 good choices:
super discrete keyfob camera:
KC-DVR COPS Portable Key Chain Camera DVR with Audio - YouTube
lots of track vids recorded with these. Cheap, very small, limited features.
MD90- more features, better low light, sound/motion activation, small and light weight:
MD90 - High Resolution Mini DV Camera w/ 2GB Micro SD Card - Home Security Store - YouTube
GoPro- more features, bigger, more $$:
Wingsuit Basejumping - The Need 4 Speed: The Art of Flight - YouTube