Here's a method for wall mounts. If you are stand mounting the plasma, I'm sure you can adjust.
Figure out where you want to sit and place your comfy chair there. Folks who intend to do a lot of HDTV or high quality DVD watching will likely want to sit closer than standard-def TV folks so as to get that "immersive" experience which is possible when you don't have to worry about the noise and other issues in standard-def TV. In my case, my eyes are almost exactly 9 feet from the front surface center of my 50" plasma when my head's in a comfy viewing position. I was targeting a range of 7 to 10 feet for watching high quality stuff so this works well even if I sit up or lean forward.
Having decided where you want to sit, get a sharp-focused light such as a laser pointer, sit in the comfy chair with your head lolled back the way you really like it, and line up the pointer along side you head along your line of sight. Pick a spot on the wall you think is precisely in the middle of your most relaxed line of vision and turn on the pointer and see how close it comes to hitting that spot. Usually your most relaxed line of vision will be a little LOWER than what the pointer would indicate. Something in between is probably a good target for the ideal center height of the plasma based on the idea that your eyes will naturally come up a bit when you are watching something intently instead of just letting yourself relax completely. There's no such thing as a "standard" height here since everyone's seating is different.
Measure the outside dimensions of the plasma (including the frame and any attached speakers) from that candidate center point to make sure you aren't hitting any obstructions. Keep in mind that the studs in your wall may be placed such that you need to shift the display left or right a few inches from your ideal location depending on the flexibility in your wall mounting system. However, you will likely be able to get exactly the height you want.
Mark all 4 outside dimensions and also the actually viewing screen dimensions on the wall with masking tape and double check from your viewing location. What you are looking for now is whether there are any problems seeing the edges of the display. Keep in mind that the front surface of the display is going to be several inches out from the wall -- 5 inches is fairly typical for a fixed mount, more for a tilting or articulated mount. If you have a thick mounting system such as an articulated arm, and if you are mounting the plasma high up, you may even want to try to correct for parallax error in your measurment (i.e., your line of sight to the plasma actually terminates at the screen, not at the wall behind it, and THE SCREEN center is what you are trying to determine.
Adjust the height as necessary to account for obstructions such as any floor-stand mounted center speaker, your toes when you stretch out, or things hanging from the ceiling. Make sure you have allowed for ventilation clearance on all 4 sides. You should also consider what's behind you that can't be screened off, since the glass in plasmas is pretty reflective. A higher screen will reflect things on the ceiling such as inset lighting. A lower screen will reflect things more directly behind you, perhaps windows, mirrors or glass fronted cabinets. As for windows behind you, it's not just daylight you want to be concerned with. Plasmas put out enough light that you may see a reflection of the plasma's own image bouncing back off the window to the plasma and out to you again.
If you intend for multiple rows of viewers to be seated, and your floor isn't raked like a theater, then you'll need to place the display high enough that the folks in back don't have to look through your head (I'm presuming you'll take the good seats in front).
There are all sorts of issues that may cause you to place the plasma at other than the ideal height. A low ceilinged room, placement over a fireplace, or placing the plasma out of reach of grubby hands of toddlers for example. It's a real nuisance to take down and remount a plasma on the wall so if you are doing a wall mount, it's good advice to make a mock up such as the masking tape idea (a cardboard box of the right size taped to the wall would be better) and leave it up for a while to see if any problems occur to you.
Also, since the plasma is really not flush with the wall like a mirror but sticks out more like a sculpture, take care if you are placing it where people might walk by and catch the edge, perhaps with something they are carrying. Locating the plasma close to a corner that people will walk around is probably not wise for example.
One final thought: If you watch TV with multi-focal glasses or bifocals, make sure you place the display high enough that you are not forcing yourself to look through the reading portion of the lenses when your head is comfortably resting.