When you say good idea, are you thinking it is unsafe, or from a decorative point - like does it look good?

From a safety stand point - most flat screens are anchored into a stud, or into the CMU, brick, etc.. so they generally are safe.

From a decorative point, most fireplaces are at the center of the room, so they pose an focal point for most seating areas. Many people love this, while others prefer to keep it on a lower console. This will be up to you and your decorative style.

That's a very frequent question that I investigated very carefully before mounting a 50" plasma. Here's what I found:

-Building code requires combustible items be no less than 6" from the opening of the fp. And with each additional inch of clearance (beyond the 6"), the combustible item can protrude by an increasingly greater distance. A quick check on Google or sites like Fine Home Building can give you code specifics, or talk to your local building inspector.

-Wrt to temps, during a Chicago winter I piled on logs and the temp never exceeded 72 degrees above the mantle, so that's a non-issue despite popular opinion. However you configuration may be different but you can do a simple test by placing your hand over the mantle to judge.

-I used lag bolts and anchors to secure the tv into the bricks and tuckpointinng. Lag bolts are typically rated at 25-75lbs each, and your mounting bracket usually requires at least 3 bolts. So don't skimp on the bolts and make sure they are secure when tightened. If need-be, construction caulk (esp the kind used for masonry work) may aid you in securing the bolts.

As the other poster said, the fp is the focal point of the room. Although the TV is a few inches higher than we are accustomed, the appearance is stunning and has a refined look.

It is fine as long as you follow the guidelines and code requirements for the fireplace install and if you are worried about the heat install a fan on the fireplace unit.