Re: 40" 1080p LCD vs 50" 720p plasma
At 10', having experience with my 50" and 42" 720p Plasmas at home, I would still go with the 40" LCD UNLESS you know you will never have any high-def signals. Our family doesn't have high-def TV (we have Satellite....no one watches enough to be worth getting HD) so 720p vs. 1080p doesn't matter. We do use it to watch movies streamed from the network, but I honestly don't notice much of a difference (we don't buy/rent movies) between them since the quality of the movies isn't great. I have watched a few in BluRay on them, and they look great...but probably would be better in 1080.
Anyways. Here's my conclusion.
Lots of HD things (PS3, X360, BluRay, using it as a computer monitor, etc.):
Get the 40" 1080p.
Lots of random, non-HD things and you want a big picture:
Get the 50" 720p.
Also, I really don't find there to be much of a debate along the lines of LCD vs Plasma anymore. I find them both equally good. Unless you will be using the screen for something that is static A LOT of the time (like if you used it as a computer monitor or gaming more than 60% of the time), then you'd probably want LCD. Otherwise, I find that newer LCDs have pretty decent black levels compared with plasmas.