yeah, It's the ambient occlusion, fake GI tool. I've been out of it tonight. I was thinking about photon mapping at the time :S Just to clear up for everyone else.

"Subsurface Scattering

All non-metallic materials are translucent to some degree. This means that light scatters inside the material before being either absorbed or leaving the material at a different location. This phenomenon is called subsurface scattering. "

so, it's the scattering effect you'd see in marble, or with skin... *crawls off to sleep*