A little over a week ago I saw Vish Matepi's video on VitalMTB with this same view. Check it out: vimeo.com/14287562
I took an older 150cm ski pole, cut off the basket end and took off the grip. Bent the grip side end of the pole to match the curve of my helmet, and screwed it into my helmet with some hose clamp type mounts. Increasing the diameter of the camera end of the pole with duct tape, I used GoPro's handlebar mount with the cam hanging upside down, facing back. Finally, added an L bracket off the back of my helmet to add a counterweight- in this case I used two Princeton Tec bike light batteries.
End result...well, you get to watch me for over 4 minutes...hence "Vain-Cam". However, I think it's fun to mix the standard POV shots up, and this was a cool test. I'm glad I didn't see anyone else on the trails, because I did look ridiculous.
Check out the drop on Fuzzy Bunny at 2:09 and Willie shredding next to me at 2:30!