I am an artist. If I include a logo for a copyrighted brand (like Coca-cola) in a painting can I be sued?

I know copyright law is a big grey area, but I remember reading somewhere that after Warhol did his Campbell soup cans, Campbells actually tried to sue but dropped the lawsuit after all the publicity they got from his work. So because of it I read that if trademarks are used in artwork for art's sake then it's Ok. Now is this true?