I bought a cheap helmet cam from Hong Kong and had a play with it on the weekend:

http://www.dealextreme.com/details.dx/sku.45576

overall, pretty happy with it - works well, is small and works with my 16GB microSD card.

Pros:
* Cheap!
* Comes with bike and helmet mount
* Good video quality
* Easy to operate and access files through microSD or by plugging into the computer with miniUSB port
* Can be used as a webcam by plugging into the computer without an SD card inserted (has two USB modes)
* Is charged by USB when not turned on

Cons:
* Mount is flimsy - does not provide a rock solid base for the camera and as such pushes extra shake into the feed (handlebar mount in particular) although you can take it apart and tighten it up
* Sound doesn't work on my unit (I don't really need sound anyway so am not going to send it back under warranty)

Overall pretty happy with it. It is possible to strap to your helmet without the mount system and I will try that further to get a solid base.

with handlebar mount

http://www.youtube.com/watch?v=XoL9NBYj1GM

with helmet strap

http://www.youtube.com/watch?v=_eMDEOtRP_k

additionally, the bottom dashboard of the videos was done with Excel 2003, Virtualdub and superimposed with Adobe Elements.

Tested and got 97 minutes of recording compared to advertised 150 minutes - will try again when I am certain it has been fully charged.