I got my 50" Panasonic plasma mounted above the mental. It was decided first day I bought the new house. So I can planned out early. I picked the lowest mental the builder offer. And I put in built in power and cable tube behind wall plus two pieces solid wood for wall mount bracket place behind wall above and under power/cable tube (so not restrict to stud). My old house, I put a 36" Sony wega TV above fire place (a large cavity just fit for Sony 36" TV). Wow, is it comfortable, I was worried before when I put my Sony up high. So here is my take:
1) viewing distance (as far as possible, i.e. > 9 foot at least)
2) height of mental (as low as possible when it still look good)
3) high back sofa, if it can pull out and recline, this would be idea (all my sofa can pull out and recline, you can rest you leg too. It can recline to nearly 180 degree if you want and they are all high back so you can rest your head on back even you sit straight)
4) Tilt wall mount (not for normal TV like Sony wega CRT)
If you have all these, go ahead. The finish is a absolute stunning. And this is a trend now for new houses. I attached 2 photoes. One is my living room and the other was builder's model home (the biggest builder in my city). You will love the set up. I have more than 30 cables behind the wall through the build in tube all the way to the build in cabinet at right. You can not see a single cable from the front. And when I use sd card slot to play 1920x1080 photoes (i.e. animal, natural view, stars), my friends would look and "wow" and glued in front of my TV.
By saying all these, the ideal viewing height is best put the TV on top of a TV stand if you can.