Using a regular XC/road style helmet isn't really great with the GoProHD and the vented helmet mount for mtn biking. I've tried it and hate it. It's also really a pain to dial in the right angle.
A) It gets hit by low hanging brush
B) It's heavy and will possibly make your helmet move around on your head
C) It's not gonna be pretty in a crash
D) It makes your action look slower
I'd recommend mounting it elsewhere. The lower the better, as long as you get a decent angle. The handlebar mount is flimsy, but it gets a decent angle. It holds up to some decently fast rough trails... I was going over big rocks at 15 MPH on a hard tail and it survived on my stem. You can get a "rollbar" mount, to mount to you downtube to get a nice shot of your suspension fork in action or to your front suspension lowers for close to the ground (doggie cam) like angle. You can get a chest mount too.
With all 3 mounts, you can make one hell of a movie, especially if you take another camera along to mount in the trees. Just change up the position of the camera every so often: handlebar (on your seatpost pointing back) for fast smooth sections, chest for twisty or technical sections, downtube for fast rough sections, and in the trees for skill/trick sections. Using a single camera angle is sure to bore viewers for any kind of lengthy movie beyond 2 minutes.