According to a recently published article, Wii players burn over 1,800 calories every 12 hours of play (average gamers play 12hr of video games a week).  From the story:

Last month, a U.K. study suggested gamers using the Wii Sports game burn more than 1,800 calories over 12 hours of play, the average time gamers spend playing every week.

That could translate into a weight loss of 12 kg, says study author and sports science researcher Dr. Tim Cable, who teaches at Liverpool John Moores University.

The benefits go beyond fitness. Another study published in the American Medical Association's Archives of Surgery suggested surgeons with a high level of video-game-playing skill made 47 per cent fewer mistakes and worked 39 per cent faster in laparoscopic surgery drills than their non-gamer colleagues.

 So, not only can you loose weight playing the Wii, it also helps surgeons perform their job better.