I have 2 x XFX 6950 2GB (reference) cards water cooled, great temps, great scores with a little overclock from within the CCC. I noticed a pretty bad flicker on both monitors when playing youtube/flash videos. I also noticed it when I was gaming (bad company 2, starcraft 2, League of Legends, etc). I tried new drivers (11.4, 11.5 + associated hotfixes), and I tried old 10.x drivers as well, with different crossfire profiles. I also tried running with 1 monitor, 1 gpu, etc.
The Fix :
Disable ATi overdrive in the CCC panel with driver 11.5 I am running solid, no more flickering! I then just use MSI Afterburner for my OC. I wrote a quick article on how to open up overclocking on there as well
http://vivithemage.com/2011/03/enabling-unofficial-overclocking-in-msi-afterburner-2-1/