Is there a standard height for wall mounting flatscreen TVs?
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").
This is an answer not to the question you asked, but to the unasked portion that was (imho) incorrectly answered in Mario's otherwise great answer. That's about where to position the speakers.
The general guidance for TV placement at eye level is great, because every one instinctively will sit somewhere where they can see the screen. Sitting relative to the speaker placement is rarely something they think about, so whoever is doing the installation needs to.
A generally accepted pattern is to mount the speakers just above head level for a small intimate space, or significantly above head level for a large space. About the only time you can really get away with them at shoulder level is if you have only a very limited number of seating positions and you have clear lines of projection from every speaker to every ear. (sub excluded.)
It's also important to keep all of the speakers in a single plane throughout the room. Getting them wildly off plane can really mess with some people's sense of balance while watching movies with heavy spatial mixes. (i.e. movies that fully use surround sound and push different sounds to different far corners of the room.)
My general rule on the height of wall mounted TVs is that it must be higher than the kids can reach, but not to high that it looks like you are looking up at the sky all the time. Of course in some cases you need to also take into account climbing access.
But eye level is also good. Most of the time my TVs are at eye level when you are standing directly in front of the TV. The distance for sitting in front of the TV also plays a part as it will naturally change your angle of viewing using this method. Further away is better.