Shadow catcher - possible?

Hi, Is it possible to have a shader/material that makes the assigned surface only visible where the shadow drops? Something similar to shadow catching in 3D applications.

I'm doing AR project where I want the virtual objects drop their shadows on realtime video feed.

Thank you.

This wonderful person (Kragh) has written a shader which is exactly what I was looking for. http://forum.unity3d.com/threads/14438-Matte-Shadow?highlight=matte+shadow