All the consumer-level helmet cam setups will use the standard composite signal to input analog camera video to the digital recording medium, which can be tape, disc, hard drive or pure memory.
For an inexpensive setup, Samsung makes a 'sports' setup which is a small, decent quality video cam with a removeable lipstick camera, linked to the body with a special connection carrying analog video and power. Records directly to SD cards.
http://www.samsung.com/products/camcord ... /index.asp
There's other helmet cams, a google search will show many.
The stuff is pretty cheap too, and any decent setup (the better ones use a well-known Sony lipstick camera) can make really good quality videos.
I plan on getting a super-compact camcorder myself, I just haven't decided which medium makes most sense, disc, tape, or memory.