I have the exact same issue.
It started with the desktop items and some video sites, then it spread to you tube's thumbnails and now it's also in full screen and in most sites.
I'm convinced it's a fault on the video card, although it happens either you have "automatic graphics switching" on or not...
I've already reinstalled Mac OS X from scratch and it didn't solve it...
Hope someone can help!
Macbook Pro 15" - 2.2 GHz i7 4GB Ram 1GB VRam - Mac OS X 10.6.8