That might be the biggest single point of compromise in any HT setup. The problem is that the sweet spot for the center channel is exactly the same sweet spot for the display.

Unless you have a front projector setup, and are willing to accept a different set of compromises to have a perforated screen that can pass sound, you are stuck with placing the center channel either below or above the screen.

This means that you will either see a dip or peak in the soundstage at the center. Sound panning across the soundstage will change vertical position. I noticed this on an older rear projector setup when I had the center down on the floor. It was especially noticeable in movies like American Graffiti where cars are constantly moving across.

I kind of find a high soundstage a better compromise. Place the center above the screen. The tweeters of the main speakers are at the highest point, and placing them slightly higher actually makes them look more balanced with the screen. Compromise a little bit and let the tweeters be below the center will get you a better balance. It also gives the advantage that the speakers can be angled down slightly. This setup works well with multiple row HTs.

The problem with placing the center above the mains is that it aesthetically looks funky as heck. Most theaters have the speakers below. The higher solution works well for small speakers, or in-wall installations, but if you are using larger speakers and the typical flat panel, it is tough to get it looking good.

There is one more possibility. You can get a pair of center speakers, and place them to either side of the screen at eye height. This sets up a brand new set of compromises though. First, unless you have a very wide room, the centers get too close the the main speakers and separation suffers. In addition, a pair of speakers, in phase will set up null spots at different frequencies in the main viewing area. For that reason, twin mains have kind of lost favor.

__________________
Living Room: Toshiba 56" 720P DLP, Onkyo 806, Klipsch KG5 mains and RC35 series center/surrounds, Velodyne 12" sub, Panasonic BD60 Blu-Ray, Toshiba A30 HD-DVD, Panasonic Laserdisc player, Motorola HD-DVR.
Bedroom: Panasonic 50" 1080P Plasma, Onkyo 605, Denon HTIB speakers, Toshiba A2 HD-DVD, Insignia BD1 Blu-Ray, Motorola HD-DVR
Misc: Optoma 720P projector and 60" screen. Basement full of surplus.