3D Objects/Projection

I am trying to port my game to CS4. It was using CS3 and some fake parallax (2D objects on 3D planes) it was done via a small scale/position modification.
In CS4, I am using the z component and failing miserably.
Collsions are based on Oriented Bounding boxes which are constructed using the (x,y) and the width/height of a movieclip. in CS4, 2 objects can have the same x,y but if they are on different z, the width/height will be smaller.
What I want is away to find the x,y of a movie clip after it has been projected on the stage (screen).
Image:
http://img512.imageshack.us/img512/8113/screenoe3.jpg