To shoot a video for the Nike iD Royale design challenge Zack Mctee built a helmet cam to hold a Canon 5D Mark II using a football helmet and some Redrock pieces. He says "it was heavy but pulled through like a champ."
You can see a video of the rig:
Vimeo: RedRock Micro Advanced: Custom Helmet Camera w/Canon 5D
And the video he shot with it here:
levimaestro.com: Zack Mctee – Nike iD (Video)