tv's should be eye level, so should monitors on your desks. This is so you don't get bad necks from watching tv/using the computer. Ever have your office evaluated by one of these chiropractor/physician type peope? I have, and it's the first thing they tell you to do. I also worked in home theater/stereo for a few years, and we always mount tv's at eye level.
Why don't you see the old floor model tv's anymore??? because they sucked, and you were always looking down at them. People seem to think plasma's are pictures, and like to hang them like a piece of art. Some think it makes the tv look bigger too....bulls*** id you ask me. In the end it is preference, but if you value your neck, it should be eye level when you are sitting.