John, I have tried vertical centers above the screen. It has not worked for me personally because I tend to place my TV's a little higher than many do. With the base of my centers over 5' in the air, moving the center of the sound source even higher away from the screen (MTM on end as opposed to MTM on it's side) tends to dissociate the sound more from the screen. Also, vertical orientaion reduces the sound radiating off the TV screen, which may improve frequency response depending on how much baffle step compensation was designd into the center speakers, but that reduction of the 'extended baffle' effect detaches the center sound from the screen even more.
Thanks for reminding me about dual vertical centers -- because of the factors above it's been a long time since I tried it. I'd certainly urge anyone to try vertical placements -- it's all about tradeoffs and the best way is to try things in your setup and see how they sound. My only point to the OP is not to be afraid of placing speakers side-by-side. The diffraction patterns that develop are certainly an imperfection to be aware of, but in everyday residential setups the benefits of multiple centers often outweigh that disadvantage.
