Optical system meaning something like the camera that's used in TrackIR, which tracks the movement of the three dots on your head?Stuff like leaning is still possible based on the roll value, but head movement tracking is not reliable enough with just the tracker (you'd need an additional optical system).
I hope for a "premium" version of this HMD that's able to incorporate something like that. Being able to lean forward, side-to-side, and up/down inside a cockpit is surprisingly useful. (Leaning forward to see the cockpit instruments more clearly, sitting up to see over the instruments better for landings). Making that integrate so that I wouldn't have to wear a HMD + Headphones + additional head tracker would be great. Although I don't know what kind of patent issues with NaturalPoint would arise.