ralexand said:
Go here for some nice explanations on many material and shader related issues in CG:
http://www.neilblevins.com/cg_education/cg_education.htm
Translucency and subsurface scattering is what you want to read first, and Henrik Jensen's BSSRDF; but there's a lot of other interesting things.
The actual math involved is probably available at Jensen's site, but VFX studios and offline renderers will obviously have different implementations and optimizations to his formula.
