I was interested by what Kayvon said about light bouncing around under the skin and looked up "skin with vs without subsurface scattering" on google images - you can really see a difference! Some of the faces before subsurface scattering seem to fall pretty hard into the "uncanny valley".

I also found the paper that includes the lips image from the upper left, which addresses a model of subsurface light movement in translucent objects (like skin!) Cool stuffff: http://graphics.stanford.edu/papers/bssrdf/