I've had my VP150 both above and below my projection screen. I had it angled towards the listener in both cases, and I liked it below the screen better for sound. Now, you are dealing with a TV. Two things here. Above the TV, and you could get some sound issues like JohnK mentioned. Below the TV, and you are putting the center channel into a "box"... Another generally bad idea (I know that there are posts about why not to put speakers into "boxes" but the handy search function may help you out there.
Tough call. I would be leaning towards putting it under the TV, angled up. Try to have it as close to the front as possible as well. Then it will be pleasing to the eye as well. If you start getting what you think is sub-quality sound, then you may have to get it out of the box and mount it up top. I've seen links to center speaker "shelves" that actually sit on top of TV sets, even rear projection, that would bring the speaker farther forward to the front of the screen.
