Since that's a plasma (I think), you can get away with fairly extreme viewing angles. Ideally the TV should be close to viewing height (however high your eyes are) or a little higher. I think a little higher is generally better because it seems more natural to look a little bit up.
Do you have a speaker system? Do you have a center channel? If so, your TV placement may depend on where your center speaker may be. Your center speaker should also be ear height (aka eye height) or slightly above. It's better to have the center speaker above rather than below, because if it's above, it can still sounds like it's from the middle. If it's below, it sounds like it's below pretty much no matter what. Anyway, the center speaker causes some issues, so that the TV should be high enough to view comfortable, but low enough so the center speaker isn't too high.