We have been playing around with tracking and PD for several years.
Camera solutions are relatively slow and inaccurate if you're thinking
about human body motion like martial arts stuff etc. Apart from camera
issues, it's also very dependent on lighting the scene and how you can
mark or detect points of interest in the visual field.
In simple systems like my Daphnia tracker ( http://softday.ie/nlutds/concept.html
), it works quite well.
For more complex tracking, combinations with accelerometers,
gyroscopes and magnetometers may help. See for example our Celeritas
demo, http://portal.acm.org/citation.cfm?id=1279780
http://www.youtube.com/watch?v=ayOOVGxzbec
(the WIMU is unfortunately not available commercially. Cheap solution,
use Wii stuff)
/Mikael