I've concluded that putting a plasma TV over the fireplace would just be unworkably high to view (bottom of TV would be 4 feet 8 inches up).
My 'hole' in the fireplace is 45.5 inches wide, 32 inches high, and 17 inches deed at the most shallow point. Ever seen a TV dropped right into the fireplace hole? (Yes, I know I'm taking fires out of the equation, unless I really hate my TV).
Rather than just dropping the TV in, I would do some type of 'build-in' so that it looked OK. Probably set up the TV on some type of rolling tray that could pull out for connections, maintenance, etc.
PS- don't have a TV right now- still planning, so I can certainly shop for the right sized TV.
Somewhat Crazy... be aware that in certain rain storms with torrential rains it is "possible" to have water come down the fireplace chimney. This has happened about 4 times with my fireplace in the last 17 years. I now have a fireplace flew cap on the stack which should minimize this. However, it is possible to have water and debris fall down and past the flew door.
I would not advise you reconsider this unless the fireplace is a fakey doo (gas or decorative) only.
I had considered this from the angle of things escaping OUT (i.e., sealing off the chimney so that warm air wasn't escaping in the winter, opposite in the summer) but I admit I had not thought about water getting IN. My thought had been to buy a giant block of styrofoam (like they make floats for barges) and cutsom cut a chunk for the hole just above the flue, then jam and calk it in. Probably a good idea to seal it off at the top, too.
Arty brings up a good point. Obviously water and HDTVs do not mix. I have not had a water problem in any of my fireplaces, but it seems to me you wouldn't have to have the TV completely inside the fireplace. You could have it out a little on the mantle on a low stand. With your opening, a nice 50" plat panel display wouldn't look bad, imo.