When I use Fg with Mental Ray, (oh, and, by the way, I figured out that the way to render the really cool sharp images was by using FG and an incandescent object in the scene) I get a very strange light source that is obviously coming from UNDER an object, but only if the object is touching the ground/plane used as the ground. Odd. Heres a picture of what I mean. I have no other light, except two planes positioned above the cube casting a bluish tinted color. PS this only happenes when a cubic-based model is used. Take a look.