Here's a cheap ($20) do-it-yourself mount you can attach to a helmet in order to put a HD video cam (or any camera that is compatible with a tripod mount) on your helmet.

I cut a mending plate (from home depot) and bent it to shape better the contour of the top of the helmet. I affixed a camera tripod mount (the top piece) to this mending plate by way of screws with locking washers (drilling the holes through the mending plate with a regular drill). I then drilled a bigger hole in the middle of the mending plate so that the camera-attachment screw would be accessible through the top vent hole in the helmet. I attached the mending plate to the helmet by rope and zip ties. Between the mending plate and the helmet I put spongy weatherproofing. The whole project probably cost less than $20. (I already had the helmet)

Here's what it looks like without a camera mounted:
<a href="https://s289.photobucket.com/albums/ll201/PolarBear1973/?action=view&current=DCP_0469.jpg" target="_blank"><img src="https://i289.photobucket.com/albums/ll201/PolarBear1973/DCP_0469.jpg" border="0" alt="Photobucket"></a>

or top view

<a href="https://s289.photobucket.com/albums/ll201/PolarBear1973/?action=view&current=DCP_0470.jpg" target="_blank"><img src="https://i289.photobucket.com/albums/ll201/PolarBear1973/DCP_0470.jpg" border="0" alt="Photobucket"></a>

With a camera mounted it looks like this:

<a href="https://s289.photobucket.com/albums/ll201/PolarBear1973/?action=view&current=DCP_0471.jpg" target="_blank"><img src="https://i289.photobucket.com/albums/ll201/PolarBear1973/DCP_0471.jpg" border="0" alt="Photobucket"></a>

and this:

<a href="https://s289.photobucket.com/albums/ll201/PolarBear1973/?action=view&current=DCP_0473.jpg" target="_blank"><img src="https://i289.photobucket.com/albums/ll201/PolarBear1973/DCP_0473.jpg" border="0" alt="Photobucket"></a>

The camera screw is accessable from inside the helmet through the top vent hole:

<a href="https://s289.photobucket.com/albums/ll201/PolarBear1973/?action=view&current=DCP_0472.jpg" target="_blank"><img src="https://i289.photobucket.com/albums/ll201/PolarBear1973/DCP_0472.jpg" border="0" alt="Photobucket"></a>

I was inspired to try to figure this out after reading about how many people have been duct taping HD cameras to their helmet.