Well there are a couple of ways to aproach this.
Most people asume that because there traditional tube display was 25inches off the floor that a plasma should be also. What gets lost is that a tube is that low because its almost imposible to get it any higher due to how big and heavy they are. The only real determining factor should be what is a comfortable viewing hight. Traditionaly the "correct hight" was eye level while sitting down. Most people agree that having to tilt you head back is a bad idea. What must be kept in mind is that the angle diminishes over distance. So, if you sit 10feet away, having the display mounted high is not as big a deal as if you sat 3 feet away.
Another thing to consider is that on a blank wall, a plasma does not look right mounted at traditional tube hights. It looks like a picture mounted to low. If there are things around the dispay it lessens this, however most that use wall mounts tend to mount the display higher.
I mounted my 42 around 45inchs from the floor...
http://img222.imageshack.us/img222/7494/dsc003776gh.jpg
At my nearest seating position (around 6feet) there is no problem viewing the display, and at my furthest (15feet) it is excelent.
It is realy a matter of preference. The only thing to look out foor is positioning it as such that a person has to tilt there head up to view. Beyond that its a matter of asthetics, and which you perfer. Check out the gallery and look at what people have done. To me, a wall mount should be done like a peice of art. If I wanted it at seating eye level then I would have just gotten a stand of some sort.