I need to make a plane in the background that is independent of any lighting in the scene. This means that it does not emit light, and it does not reflect light, but it is visible. It is so I can create a green screen.
You don't need to use a shadeless green material to simulate a green screen. You can use an object to create a mask to use in compositing. If your not compositing in blender then these can also help with other applications, by saving the frames with alpha channels you have already cut out bits that you want.
In cycles you can use a holdout shader. The blender wiki describes the holdout shader as -
A holdout shader is useful for compositing, to create a "hole" in the image with zero alpha transparency where the object with this shader is located.
Blender Internal has a similar feature by choosing Mask under Transparency options
Mask Transparency is a special kind of transparency. Instead of making objects transparent/translucent, it hides the geometry behind it.
This is obviously easy and helpful if you render to an image format that supports alpha.
Another option that is available in both cycles and blender internal is Pass Indexes.
These can be set in the material properties for Blender internal -
and cycles -
Or you can set a pass index for an entire object -
To access them you need to enable Material or Object Index in the Render Layers -
Then in the compositor you use an ID Mask node to get an alpha mask of the material or object -
If you are rendering out to file for another application then you can use this mask to cutout the section you want and save with an alpha channel the same as when using a holdout material.
The pass index supplied to each material needs to match the index used in the ID Mask node. You can apply these to multiple objects and materials at the same time and can use more than one index id to handle each mask separately.