Personally I wouldn't do it.. not only the heat, but like with my fireplace, when you first light it, there is always a little smoke that escapes until the flu heats up to help pull it out.. hence the reason you see fireplaces that get used a lot with a little bit of smoke colored mantles.. I wouldn't want that on my TV.. they cost too much to do that to..
Get a DVD of a fireplace and enjoy.
I agree. I have mine mounted higher than I've had in the past but I haven't noticed any issues with it. My plasma would be too high above my fireplace.I wouldn't do it just because that's much too high for me. You generally want a TV at eye height (while sitting).
I agree with others, its too high (32-36 or so is usual bottom of screen to floor distance) and the potential issue of a fireplace, I think many probably dont actually use the fireplace they install a unit over. Maybe some have gas but I cant imagine any installer (and lets assume a majority farm out that installation) would think a wood burning frequently used fireplace would be a good idea.
I've seen this a lot and it always puzzled me why people would want to cock their head up to watch tee vee.
Murray