Create a plane that renders nothing behind it? I'm kinda following this tutorial, but using cycles instead of blender internal. Because I'm using cycles, I had to manually set up the nodes and render layers. This image is what i'm supposed to have currently (from the tutorial):



And this is what the render result is for me:


I have transparency checked. This is my node setup and render layers:



And these are my layers:

Layer 1: 
Layer 2:
Give the plane a holdout material. This should make it block everything behind it and yet be completely transparent.

I will try to update the answer with screenshots soon.