Here is a great answer the the question whether to mount a TV over a fireplace. As the author says, heat is not as much as an issue as the optimum viewing angle.
http://www.hometheater.com/content/flat-panel-over-fireplace

Another issue is that the flat screen is not a piece of art and actually is an unattractive piece of technology along with all the boxes and DVD players associated with it. So, some thought needs to be considered when designing a room that is for entertaining. Design for the equipment to be built-in as most of the TV furniture is either bulky or as unattractive as the equipment. Custom built cabinetry that is an intricate part of the whole room design is best.
There are some wonderful looks and designs here on Houzz so check them out.

My Thoughts