Mounting a TV above a fireplace
It is a bad idea. TV screens are designed to be viewed basically level from your eyes as you sit in your TV room. If you mount it above the mantel you will forever looking up to see the thing. 

The heat thing is also a concern for electronics as you have mentioned. Raise the temperature some and in the best case you will age certain components and shorten the life of the TV.

You would also have to deal with the disfiguring of the brick face of the fireplace when you find that it is a less then optimal place for a TV and decide to take it down.

I fail to see why this is such a fad idea to put TV's up high on the wall.