Is there a standard height for wall mounting flatscreen TVs?
Eye level

The center of the TV should optimally be lined up to your eyes. This will change based on the purpose of the TV. In the kitchen, our TV is about 6 feet off the floor over the counter top because we do a lot of cooking / baking; so we're standing when watching TV.

In the family room, take this measurement when you're sitting on the couch. 

Same thing with speakers: place surround sound speakers at roughly the height of when you're sitting down from the floor to your shoulders (for a 6' individual, that's about about 26").