I made the leap to Lion 10.7, and can conform that the screen issues are gone. I don't know what changed, or what was fixed, but something was done right. I've even updated to 10.7.1, and the screen issues are no more.
I am using my macbook 10.6.8 with latest updates with external monitor. Have same issue.. but there is easy step. Open System preferences>Display and Uncheck Automatically adjust brightness as ambient light changes.