An example by a Japanese artist can be seen here (http://www.youtube.com/watch?v=YE-AnfhqZqg)
And he also has an xsi shadertree here (http://artifacts.sakura.ne.jp/sakanaya/2007/10/toon_rendering_retouch.htm)
It seems to be a good way to prevent weird or unpleasent things in cel shading.
I'd like to do the same thing, but In maya, and preferrably use a texture map instead of vertex map. I think the basic concept would be to somehow try to add the values of a black and white map to the surfaceLuminance node, which drives the v value of a ramp node in maya, but I've yet to get it working. I don't have too much experience on shading in maya, so any help is appreciated :)