Mount an LCD TV to 30 year old masonry fireplace
The main thing to worry about when mounting a TV on a fireplace that you might use in the future is heat damage to the TV. What I did before mounting my TV is get a good strong fire brewing in there for an hour or so, then get a good thermostat reading from the fireplace where the TV would be mounted. Make sure you then check that temperature against the allowed temperature range for TV operation and storage before you start drilling holes in the masonry.

Just because your TV operates on the mantle now does not mean it will be OK after it has a solid connection to the bricks, without that nice comfy air buffer in between.