http://www.boingboing.net/2008/09/06/google-launches-sate.html
In a speech last month to a security conference in the UK, Stoica explained that by using shadows you can read the length and rhythm of someone's gait and do an identification, even from above. He has written software that isolates the shadow from video, and adjusts for time of day and camera angle to deal with elongated and foreshortened shadows. Stoica shot video from the top of a six story building to test out his software and was able to get usable gait data on his subjects.