I ran across this article (below) a few years back.
Maybe it will give you some guidence. It's pasted to you, so I am not sure how it's going to transfer.

Macey

Plasma TVs Above the Fireplace?
Reviewer: Phil Connor

Copyright 2004 PlasmaTVBuyingGuide.com. All Rights Reserved.

We receive a lot of e-mails asking about the possibility (or even plausibility) of placing a Plasma television or plasma monitor above a fireplace.

Living room configurations often put the fireplace as the central focal point. Since we all know that the real focus of your living room is the television, we hit upon the question of integration -- how to merge the logical focal point of a living room (the fireplace) with the actual one (the TV). The introduction of plasma displays has made such integrative gestures real possibilities. Mounting a plasma display above the fireplace is a great space saver, not to mention an attractive way to display the panel. This is a very common application for plasma TVs today.

Will the heat from the fireplace harm the plasma TV?

There are a few precautions to take before hanging a plasma screen TV over your fireplace. First, take a thermometer and tape it to the wall above the mantel, in the place where the plasma TV will reside. Then build a fire, and let it roar awhile. Now, check the thermometer. If the temperature is above 90 degrees F, then the area is receiving too much heat -- either escaping from the front of the fireplace and rising up the face, or radiating through the chimney to warm the outside wall -- for a plasma TV to hang there without receiving some modicum of residual damage. It would not be advisable to run the plasma for a long period of time while its environment is this hot. Though the Plasma TV has its own cooling system, the surrounding heat will force the Plasma display to work harder to cool itself, thus shortening the overall lifespan of the unit over time. A Plasma TV may still be run for short periods of time without harming the unit (even with the environment heat at 90 degrees F or above).

If the plasma display is not turned on while a fire burn in the fireplace, then the unit will not be harmed at all even though the temperature surrounding the unit may be warm. Of course, this might not be an option for people living in places where winter lasts seemingly forever, and fires are an everyday fact of life. In such cases, you should be advised to monitor the amount of heat your plasma display is exposed to while it is running.

Can the Plasma TV be mounted onto a brick fireplace?

Yes, cement anchors may be used with the wall mount to secure this application. Again, the amount of heat to which your plasma is exposed should be a concern here, as some brick fireplaces can radiate quite a bit of heat.

Will the Plasma Display TV be too high? Can I tilt it?

Even though plasma TVs typically have a 160-degree viewing angle, tilt wall mounts may be purchased if the mantel is high. A tilt wall mount application will allow the user to tilt the unit from up to 15 to 25% vertically (in this case, downward). With this mounting installation the user will have the choice between a flat or tilted affect while adding only 1.75 inches depth to the plasma TV. A flat wall mount can add as little as 1.25 inches to the depth. So, the difference is only half an inch.

Can I just place the plasma on the mantel?

A typical table stand for a Plasma TV is between 4 and 6 inches in depth. If your mantel can accommodate such a plasma TV stand, you have it made.

Might be a good thing that you are looking at Mendota: They seem to allow you to mount the mantles lower than on some others, implying less heat up there...

Plus, on many mendota models you can 'box in' a space over the fireplace, meaning you wouldn't need to commit to a plasma, you could do a DLP or similar. You still wouldn't want to get it super hot, but I *think* they'd be less heat sensitive than plasma, plus it would be easier to create some moveable art to disguise the TV when not in use.

Final thought: I'm doing a TV over a fireplace, probably a Mendota. I'm not too concerned with the heat issue as I don't imagine I'll be using both at the same time very often. The fireplace will be for social times, or quiet contemplation of a good cognac on a snowy evening. Those are times when I don't want the TV distracting me, so its not a sacrifice to have to choose between the two.

We do a lot of fires in the Winter while watching a good movie or two every night so for us we decided against mounting our plasma above the gas fireplace. DH didn't want to take any chances with the heat and the tv, can't really blame him, big investment. I do think it looks extremely sharp to have one above the fireplace, especially with concealed wiring (I do know one couple with the tv above the fireplace and all the tv wires duct taped together draped down the side of the mantel - not a good look) :)

The majority of people I know with tvs above the fp don't use the fp (or hardly ever).