Has anyone written any code to translate a screen point (x/y/z-buffer-value) into world co-ordinates? I had assumed that's what modelX/Y/Z did but it turns out they don't, so is there any other way to find out?

I do have one method but that requries me trying to work out how to create a vector from the camera position, to the plane of the screen, then to do an intersect test with every single polygon drawn on screen, which becomes unusably slow for any even slightly complex scene.

Basicly what I'm trying to do is draw a 3D scene, then draw an obect resting on the surface of another at the position that the mouse pointer on screen.