The increasing importance of celebirty in politics is certainly not positive. However, I don't think the kind of thing you're discussing in this thread--actors making comicaly righteous speeches in interviews and at awards ceremonies--is the most dangerous aspect of celebity. Far more dangerous is when politicians rely on their fame to win elections. George Bush would never have been president of the U.S. if he hadn't had a famous father. Hillary Clinton would never have become a Senator if she hadn't been married to one of the most famous men in the world. Ronald Reagan, Arnold Schwarzenegger, and Jesse "The Body" Ventura all rode their fame to victory, and all were incompetent lightweights. A proven ability to govern is much more important than having a famous name or having appeared in a few bad movies.
That said, I did like Clooney's Good Night, and Good Luck. I've never heard him hold forth on politics, and I don't really want to, but I think it's great that he's taking on political issues in his movies. With all the stupid fluff films that come out every month, I admire people who try to make movies about issues that are really important to us. Storytelling--in the form novels, movies, and plays--is one of the most useful ways to work through political issues.