They used a commercial 6DOF head tracking solution from Naturalpoint called TrackIR 4 Pro. The manufacturer promises:
Experience real time 3D view control in video games and simulations just by moving your head! The only true 6DOF head tracking system of its kind. TrackIR takes your PC gaming to astonishing new levels of realism and immersion!
The tracker is supposed to work at 120fps, and Jyri’s video demo was convincing, so it looks like an interesting and cheap mocap tool.