Update: I've finalized the cel shading tech, a lot of old issues have been solved.
Here's the up-to-date tutorial:
Use this to get the grayscale data to cel shade. For example, use a Step or Smooth Step on it.
Use this to get the world lighting color.
Use this to get the additional lighting color (point lights for example).
Plug this into your textures like so.
Everything else is up to you, the shader artist, to do. Whether you're going for a painterly look or a comic-book style, it's all accomplishable with this.
