Im pretty sure the wall in this case(the fire breast) would be strong enough to hold the weight.I know what your saying though about internal walls etc.Ive seen a few with wall mounted plasmas/lcd that look like they are about to fall down.
I think as soon as you go over a 32" then you need to choose carefull where you mount them.
The last think you want is for you lovely new tv to end up on the floor in pieces.
