I also see this issue - the screen goes black for 1/5 of a second. I'm using beryl on ubunty feisty. 2.6.20 kernel, Xorg 7.2, nvidia 9631.
Sometimes the flicker freezes the screen, but I can still move the mouse (and kill X). Sometimes it freezes the whole system completely. It seems to me that a grabbed focus + flicker makes it worse.
Also seen here: http://forum.beryl-project.org/viewt...hp?f=36&t=3025
Oh, and I'm on a Dell XPS M1710 laptop, with a Go 7900 GTX.