For my first "hacked together" HT, I had my center channel about 2 feet to the side of the TV (on top of my audio rack) and was always surprised how far I could go before it became distracting.
I now have a Polk Audio CSI-5 sitting one foot below my screen. The CSI-5 can sit flat or flipped over so it sits at about 15-degree up angle. Technically, it is closer to the center of my screen than the two speakers attached to either side of my plasma monitor. So, if you are looking for a speaker like that, I think you'll like the results.
I also experimented with the Panorama setting on the 990 and found that at "1" (in my case) it sents just enough of a hint of the center channel to the main speakers that the sound for the center channel seems to be coming from the entire screen without "hearing it" on the main speakers.