Unity doesn’t do negative lights 
so I’d like to tag lights and have my shader remove light from these tagged lights.
How do I read a light’s tag on a lighting shader?
(by the way, the new lighting/surface shader system is fantastic ! It reminds me of PRMan, thanks Aras)
sounds like you just want to use layers actually
I want to use the light tag to negate its contribution in the lighting shader
Well the first and most important thing to keep in mind:
Shaders don’t read anything. They process vertex or pixel data, but thats it. To them there is nothing else than that, no tags, no objects no nothing.
Tags are used for the engine only to define the order in which what is pushed into rendering, thats it.
So if you want to have a negative lighting, you would have to create a postfx (likely replacement shader) and switch between positive and negative lights in there, then render them into a buffer and apply the resulting light data negatively against the original backbuffer.
That would at least be my starting base.