I did this over this past weekend.
I put the ln52a750 from Samsung above the fireplace.
BTW...this is the perfect time to make the argument to the wife that you need the big screen to totally cover the niche.
Otherwise it will not look right.
If you have any carpenter skills at all, this is actually pretty easy.
The builders used studs to corner the niche so you have wood on all 4 sides to mount your brace into.
You do not need to put sheetrock over the brace. Just make sure that the brace is flush with the wall. That way you can take it out should you ever move and you can, should you want a little overkill, drill holes all the way
through the beams in the brace and run 1/4" bolts with washers through. This will be far stronger than any bolts you will ever drill into the beams.
But hey..when talking about a TV that is both fragile and cost thousands...a little overkill does not hurt.
If you do a search for " Wiring questions for Niche above fireplace ", you will see the thread that I started last week.
My biggest concern then was getting the wires run however. In that thread, I spelled out how I did that.
The brace for inside the niche is not hard to make.
I did take it a step further also. About 7" from the bottom, I created another section of the brace.
I wanted to be able to set my center speaker under the TV. I also wanted to be able to access the niche in case I ever needed to do something like reset the surge protector, etc.
Right now, I am building a speaker screen that will cover this bottom section and make the whole setup look clean over the fireplace.
I created 3 beams in the top part of the brace and made sure the middle one was perfectly center. This allowed me to use it as a reference point.
Also..make sure you get a wall mount that tilts. It will be far more comfortable to view if you can tilt the TV. I got a Peerless mount.