For this quick video I spent about 30 minutes playing around with the exmaples provided with the processing SDK, you can get a copy from processing.org

[FLOWPLAYER=motion_tracking_demo.mp4,320,262]
UPDATE 14/02/09: I forgot to upload the source code for this, it is now available here: https://svn.blog.oneiroi.co.uk/branches/processing/Motion_Tracking/

svn co https://svn.blog.oneiroi.co.uk/branches/processing/Motion_Tracking/