Thermal transfer from the phone to the case would have to be in direct contact for it to be as effective as we want. Think of a CPU cooler. The cooler is in direct contact of the chip(ie the phone.) thermal energy transfer is very inefficient through the air space inside the case so the case would have to be touching the phone to act as a heat sink.
When I was thinking of a polycarbonate case, I was hinging on insulating the phone from heat, which it does. But doesn't allow it to vent. Aluminum on the other hand will pull heat from the phone if designed properly, but at the same time is conductive from the envioment since you are effectivly putting the phone inside a waterproof greenhouse.