We had our plasma over a radiator shelf above a radiator for more than a year with no problems as the heat from the radiator flowed up from the front of the shelf and into the room. It had no effect on the temperature at the screen. I have 3 lcd computer monitors on desks with radiators underneath, again no problem as yet. Mounting a screen directly above the radiator without a shelf, desk, cover diverting the heat flow would be asking for trouble.
HTH
Dave