Difference Keying?
 

Is it possible to snag tripod footage of a static background...and footage of a subject in front of the background...and generate a matte dependant on the difference of the underlying/overlying pixel info? Using a method like this could eliminate the need for a green screen.

Is there already a procedure like this? Or...has it been tried/failed?

I'm new to the forums! I love the info here! Thanks a lot!

Yes, that is possible. If you know the movie 405 ( www.405themovie.com ) they used a very similar technique to
clear out a busy freeway. It all depends on what the quality of
the images is, what you want to do, the programs you've got
(and the talent ofcourse) and how clean your backgrounds are.
Seperating someone from a busy detailed background is very
difficult compared to a even background or green/blue screen.