Actually, I am a physicist, and kgarner is technically correct. The quantity you should all be looking for is *emissivity*. The derivation of the blackbody law assumes equilibrium, which means the composite system isn't changing certain properties (e.g., temperature). If a body is in thermal equilibrium, it must radiate as much heat as it absorbs (otherwise it would be warming up or cooling down). In either case (absorption or emission), the RATE at which heat is transferred depends on the area, the temperature, and the emissivity. You can look it up, but I would bet that the matte black finish will have a higher emissivity than the glossy white finish.
However (and this is the most important thing), thermal transfer due to radiation goes as T^4, so it really only matters when the radiation is coming from somewhere really hot (like the sun). If your laptop is inside your room, it won't matter which color you have. What will matter is if the thermal paste inside your MB was properly applied.
-RendIt