This isn't Xbox related, but it's absolutely worth mentioning:
A man named Greg has shed 41 lbs by playing World of Warcraft. He placed a stationary bike in front of his monitor and got addicted to the game. So, every hour that he was playing the game, he was riding the bike. It should also be noted, that Greg lifted weights and followed a strict diet, but according to him; WOW made the biggest difference.
"I really think the Warbiking had a LOT to do with how well I did, and how much I lost. I have done over *100* hours of cardio in three months, which is wild. There is NO WAY I could have done that without Warcraft … it's [sic] ability to suck me in and make me ignore what I'm physically doing is amazing, and made this possible."
Props, Greg.
Check out Greg "The Warbiker's" blog here