Just bought one a couple weeks ago from helmetcamera.com
Took it out on our skiing trip, but haven't tried it on the dirtbikes...yet. :)
Purchase the external microphone for better audio.
The whole thing is very easy to hook up and use, just make sure you find a safe place for the camcorder. If you buy the helmet camera kit, it comes all nicely packaged in a plastic foam-filled case. There is a separate battery that runs the camera and microphone, so you don't have to depend on your camcorder battery to do all the work.
Now all we need is a Firewire card to upload video to the computer!