We are thinking of mounting our tv over the fireplace in the living room.
This is a bad idea, imho. It isn't the heat of the fireplace, or the weight, that I would worry about, but rather the effect on your neck. If we were meant to keep our heads pointed in that direction for extended period of times, the underside of our necks would be a pretty part of the body.
Ideally, televisions should be mounted or situated so that the middle of the screen is at eye level
when seated. That's one reason why, with all the big screen televisions, it was necessary for furniture manufacturers to start making specific stands, much closer to th ground than ever before -- the screens were larger, so the had to be placed closer to the floor than older televisions typically were placed.
Just from a design standpoint, televisions, though so much effort has been made to make them more attractive, are not designer appointments. They're not pretty; they don't fit with any decor other than if you were decorating your home to look like the Starship Enterprise. The best design approach, vis a vis HDTVs, imho, is to make the thing fall into the background as much as possible. Ideally, someone would walk into your living room and say, "Oh! You
do have a television. I didn't see it initially." (Okay, that's not likely, but again, it's an ideal.)
At
worst, I wouldn't mount a television any higher than with the bottom of the television at eye level when seated.
Here is another reference you'll want to consult. It indicates how far from your eyes your television should be placed. That consideration, itself, may invalidate mounting it on the wall:
http://hdguru.com/wp-content/uploads/2006/11/hdtv_distance_chart.pdf