Correct me if I'm wrong, but I believe I read elsewhere on the forum that plans to upgrade CT so it can use background vids have been abandoned or put on hold.  This is a shame, given what a compelling feature that would be, but maybe here's a solution.

I know I can easily add a green color to the background of CT videos.  The challenge is converting the green to transparent.  If the RealIllusion team doesn't want to code this feature, why not tell us who already has? 

I'm looking for a simple app dedicated to converting the green screen to transparent.  I'm on a Mac.  Any suggestions?