I hope you don't mind me deviating from your question a bit. Some years ago I saw a helmet which detected the wearer's head movements and transmitted them to a remote robot controlled head. Cameras on the robitic head then transmitted back what the robot could "see". The idea was that you could have a remote telepresence.
Rather than using a camera to detect head movement you might be better to go down the helmet route. You could either build your own montion sensing helmet or perhaps hack something like the Oculus Rift Virtual Reality Headset. I presume it already has an application interface of some sort. http://www.oculusvr.com/
Unless there is a reason why you have to use a camera I think the helmet offers more posibilities.