I'm surprised no one posted this here. xD
It's official, and it's based on the 2007 Olympics. (^^) And no worries about the game being glitchy/somewhat sucky (like a lot of Sonic games have been lately). Shigeru Miyamoto will be overseeing this project; since Mario is his baby (and he's a game making legend), he won't let the game suck.
Also for your entertainment (though I doubt the game will be this "graphic"):
(Newgrounds.com) - Mario vs. Sonic