Recommendations for inconspicuous helmet-mounted video camera
I bought a DOGCAM Bullet HD Wide camera, on the basis of a decent camera at a good price. The Contour HD is a nice camera but too expensive for me.

The DOGCAM Bullet is compact unit, 80mm long and 22mm diameter. Waterproof so strapped to my helmet for kayaking, on the bike I tend to strap it to the bars. It'll last a couple of hours on a battery charge, it's an internal battery which is charged via the mini USB port in the back of the camera. Plugging in via the USB it just appears as a removable flash drive on the computer to copy the video's.

My only real gripe with it is the on/off switch, there's very little tactile feedback to know if I've pressed the button. It does have a small LED at the front to indicate when it's recording or the memory card is full but in daylight conditions it's not the easiest thing to spot. When it's on my bars I really have to lean forward and squint to see the LED. 

There is a fair review and example video on Cycleseven