(This question is among a batch of questions that were asked at the DisplaySearch HDTV Conference 2005. For more information, you can click the logo below.)
Ever hung a big , heavy mirror? I helped my neighbor hang his 42" LCD over his fireplace on 100# test picture wire over bolts in two studs. It's been there 2 years- his mirror has been up 4 years. Makes it easy to get to the connections - just lift the bottom away from the wall (very carefully), and fits tight against the wall. So you need a fourth option in the poll - none of the above.
Scary huh? He bought a mount, but it wouldn't work with his nonstandard studs , so that was his solution. I did make him put the mounting bracket on the back of the LCD to distribute the load on the picture wire to all 4 mounting bolts instead of just connecting it to bolts in the top 2 holes - cost him his refund for the mount, of course, but I could only take so much.
As for mounting over a fireplace, no problem. You need a mantle to deflect heat - other than that its a matter of viewing angle. I had my fireplace custom designed with the top of the mantle 53" above the floor. I've had a 42" plasma and now a 45" LCD mounted 5" (at its base) above the mantle on an articulated mount attached in a 12" deep recessed area that fits the flat screens. Viewing angle from 9' is great - don't even use the tilt function.
Generally I would rather have my display at eye level. But with the fireplace as the focal point of the room with seating arranged to enjoy it's warmth and ambiance, it seemed to me that if we were going to have a large tv in the room, it should be in the natural focus of the room, i.e., over the fireplace. Its worked well, and it's nice to enjoy the HD and fireplace the same time.
Your comment about the mantle deflecting heat was interesting. I assumed (I know) the heat which would affect the tv would come from the fireplace wall behind the tv. not from below. Or, is heat from both areas a problem?