In the old days the fireplace might have been a focal point for a room.
The thing that the seating was designed around.
Today, it's often the TV.
So, what about this....?
Brick up the fireplace and mount the plasma in the middle of the wall at a comfortable viewing height.
Then, when you have the urge for the fire, light up a DVD with fireplace-video and enjoy.
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Ideally the center of the screen should be at eye level. Unless you stand to watch TV, the only thing above a mantel should be a family picture.