The Capitalist's Bible, also known as the Wall Street Journal, gives a backhanded compliment to the Wii today saying it gets people off the couch, but "is turning out to be more exercise than some players bargained for." The article explores how people are beginning to develop -- wait for it -- "Wii elbow." One after another the WSJ serves up another poor soul who has been exerted by the Wii-ness. For example, Ryan Mercer lifts weights during the week but Wii Sports' boxing is pwning him, "I was soaking wet with sweat, head to toe -- I had to go take a shower." The following morning he was so sore he had trouble getting his shirt on.
Nintendo's name-dropping spokesperson Perrin Kaplan is quoted in the piece saying she hasn't received any complaints about soreness. Serving back WSJ's backhanded compliment she says, "It was not meant to be a Jenny Craig supplement ... if people are finding themselves sore, they may need to exercise more."
So the answer is to train up before you Wii and you won't get sore, but please remember you can always play the Wii like a toolbox.