I normally wouldn't have bothered to add anything more here since so many others have already weighed in with the problem, but I've got a slight difference to add in. While the screen is duplicated on all three monitors, the tertiary monitor also shows one other symptom. The background (lighting) is not being updated. I'll pop a downsized screen cap to:
http://webpages.charter.net/rlsimon/beta7m.jpg
While it's an interesting effect, it does detract from the overall look a bit.