Proximity sensor?

[Toby Paddock]

How about using a *Polaroid Sonar* ultrasonic rangefinder?

That would give a distance indication so you could define 
how close you want the person to be.
It wouldn't just respond to a change like an infrared sensor.
Also it wouldn't matter what is happening in the background.
After the show you could turn it into a CV controller.

Polaroid OEM:
http://www.polaroid-oem.com/american/new/usp.htm
(may be an old dead link, and their prices were 
kind of high anyway)

Wirz Electronics:
http://wirz.com/sonar/index.html

Modifing one from a scrap camera:
http://incolor.inetnebr.com/bill_r/polaroid%20sonar%20mod.htm

 - -- -  Toby Paddock
http://www.seanet.com/~tpaddock