There are a few things to consider when thinking about mounting a tv above a fire place. First is what others have mentioned. It is an odd height and angle to view a television. If you are sitting far enough away, it may not matter. But if you are relatively close, this could pose a big problem and your enjoyment would be greatly decreased. As far as the installation is concerned, hiding the cables in newer construction isn't too difficult, as most fireplaces have an empty space, usually 1 to 3 inches away from the flue. As long as you're drilling into that empty space, hiding the cables won't be a problem. Support for the mount can be done to a stud behind the mantle area.
Hope this helps some. Fire places aren't the ideal place, but when done right, can look nice.