From the school teachers perspective...I see far too many kids who idealize athletes. My kids are sure they are all going to be multi-millionaire professional athletes even if they are not playing on the high school team. For that reason, the Michael Phelps thing is a big deal. My students would think MP smoking pot is awesome. Just leads to the idea that the rich can do whatever.
From a parent perspective...I don't think a young kid smoking pot is the end of the world. I will certainly tell my kids not to, encourage them that some behaviors/actions have consequences that might not even be immediate (like pixs on myspace, etc..) I'd be far more concerned if my kid got a DUI. Then, their stupid behavior is putting other people at risk.