Originally posted by: ActuaryTm
Actually, by very definition, a heatsink is merely any device that aids in dissipating heat away from a given source - regardless of the material of said source. In this case, the conductivity of plastic (thermal constant < 1) is indeed not wonderful for the transfer of heat; however, as most know, aluminum's thermal conductivity is quite acceptable (thermal constant ~200). Even without the fans nor a true "bond" between the two materials, the ample aluminum surface area (in addition to the ventilation under the aluminum) would allow for quite a bit of heat to be absorbed and then dissipated. The fans only help this.
It's shear physics, very plain and simple.