Do you mean after a few minutes of non use the screen dims? Only to return with a touch of the mouse or a button?
its a bug. Apple has already acknowledged this. I believe that their workarounds do not work.
http://docs.info.apple.com/article.html?artnum=304092
Maybe "bug" wasnt the right word. It only affects Intel based laptops. Its been a "problem" or "issue" for a few months now..
"Date Created: July 24, 2006"