Cross section of plane

Hi all! I am in the need of a shader or script that enbles me to make a cross section of just about any 3d object. There are a lot of shaders available that could to this for many cases, but i need to be able to make a cross section of a plane, or a cube with sides made out of planes. That is, i need to make a cross section out of objects with zero thickness, and still show something indicating that there actually is an object there.

Is there anyone out there who has been doing something like this, or have any ideas of how to do this?

Cheers joel