My twin GTX 460s either flickered or werent doing proper SLI. I found an SLI profile update for the C2 demo here:

http://www.nvidia.com/object/nvidia-multigpu-SLI-profile-update-v7-driver.html

applied it, and crysis2.exe and crysis2demo.exe both now show up with an "Nvidia Recommended (Custom)" SLI setting. My cards are now working perfectly, and my framerates went way up (90fps @1929x1080 advanced).
I havent noticed the flicker again either, but I KNOW it cured my GPU underutilization!

Or you could just try more recent drivers, such as:

Beta 267.24, 267.31 for GTX 580s
Beta: 267.59, 267.60, 267.66 <- Good performance increases and almost no flickering.
Beta: 267.84, released today for the GTX 590, <-More performance over 267.59 series.

People do realize their is a Support forum for this kind of thing yes?

Thanks, I'll check it out -- I thought these applied to 5xx series only? I'm not sure I would throw beta drivers into the mix -- I really just want to play the game, not dink with drivers. I was overglad to find this simple fix.