I don’t see that this has been posted already, so I figured I would draw your attention to this xkcd blog post wherein Randall takes on the old chestnut What would happen if everyone on earth jumped at the same time. (He mentions the Straight Dope column in his answer.
Munro sets up his question with everyone on the earth gathered together in one area, so the question is a bit different, but it’s fun. The interesting part is not what would happen when we all jump, but what happens afterwards when everybody tries to get home.
Key quote: Within weeks, Rhode Island is a graveyard of billions.