There's a photo in the thread that Karve linked to above, but here it is:
Put bamboo on top of helmet, duct tape down. Tape a small rock to the back, just behind the helmet. Attach the gopro to the bamboo using the handlebar mount (you might need some packing material, depending on the diameter of bamboo!). Ride
The coke bottle was a later addition, as in the setup pictured the gopro was too low, which meant my mate couldn't see where we was going! It was just wedged in place between the bamboo and helmet to lift the gopro up a bit, you can see it in the crash at the end of the video.
It wouldn't be just 'sitting out' that would concern me, if you're talking about a completely solid metal rig bolted to your lid, I'd be more worried about ever walking again in the event of a crash! When I was looking into it someone summed it up well by saying 'the awesomeness of the shots it gives is only surpassed by it's ability to maim'. When you ride with all that weight pivoting around your neck, you'll quickly see what he meant. One run with it on was enough for each of us!
Chris