Shader Question

I need to write a shader that culls objects that have been given a designated layer. Is this possible?

You could modify the shader in this answer to give 100% transparency to them by having the second texture set to 100% transparent when the layer value is set.

You can do that in the Culling Mask setting in the camera settings
(see here:http://unity3d.com/support/documentation/Components/class-Camera.html)