I will give a much more thorough update on SIGGRAPH 2008 later in the week, but for now, have a quote:

Ridge and valley lines are drawn at the local minima/maxima of intensity, in the direction of the max Hessian eigenvector

I love this place… this is normal, and makes sense, and can be applied to art to boot. That snippet was from a talk about drawing lines on 3d geometry to better approximate the ways artists use lines, as opposed to the basic toon-shading approaches we have now.