I plan to mount my 40" lcd tv over the fireplace. there will be 21" from the top of the fireplace to the bottom of the TV with a 7" deep mantle between. I have between 4-5 fires a year. Will the heat from the fireplace damage the TV?

I would refer to the owners manual to see what temperature the unit can handle.

In addition to the possible heat, which all depends on how well your FP is drafted, the soot will collect on the unit.

Me personlly, I would not have my TV above a fireplace.

The wood smoke/soot will definitely not be good for your new TV set. Would recommend putting the TV elsewhere unless you don't plan on using the fireplace ever again. Wood fires are very dirty expelling tiny dirt particles into the air that your set with grab as the TV needs to be ventilated and the particles will latch onto the TV components inside the box.

Although the tv's look great mounted above fireplaces in the interior designer's photos, etc. the placement is definitely not TV friendly. Pick a different spot far enough away from the fireplace or you'll be replacing the TV every few years or so.

As mentioned by twinkie... it's not so much the heat as the soot and smoke. If it's a wood burner, I recommend not doing it. If it's gas, then I wouldn't be to concerned. I've mounted plenty of TVs above gas burners.