Can an HDTV screen be mounted above a gas fireplace?

or will the heat from the fire damage the screen? Thanks

It is definitely not the safest place to mount the TV. But since it is probably the best spot you found and If it is a closed fireplace, my answer would be maybe it's oke to do this. But read the User's Manual of your TV first. There should be a part about the environment (temperature, location etc) in which the TV can be used. Just remember that safety should always be your first priority.
No it should not be. An HDTV should not be mounted or placed near any place of significant heat. This could damage the TV itself and undoubtedly void your warranty. It could possible melt solder, diffuse colors in Plasma TVs and sometimes if it is a Plasma TV, the tilt of the screen, if it is too great, can cause some of the gasses inside the screen to physically rearrange and it can mess up your screen. Bottom line is, NO, do not mount your flat panel HDTV over your fireplace.