Create "Floor" plane in cycles? I've got a scene that I've motion tracked and it seems to be working well. Then I used "Setup tracking scene" to create a cube layer, then a floor layer that would only exist to act as a shadow catcher.

When I render this in Blender Render, such as Blender Guru does in his tutorial video, it looks fine.

However, because of what I would like to do with this scene in the future, I would rather use cycles instead of blender render. However, I don't think the nodes are set up right or something because when I render it in Cycles, the floor layer still appears.

I would like the floor layer to only act as a shadow catcher AND (this is hard to describe) a layer where anything behind it would not be rendered. So like if I had a cube that stuck through the floor layer, only the part that is in front of it would be shown.
I should also note that I have no experience with nodes, so that's probably the problem.
This feature is now included.

Select the shadowcatcher object, go to the object properties and check Shadow Catcher under Cycles Settings.

It will render a shadow but mask the object behind it.