There's been numerous rumors from a few sites saying a May or June date. I'm not saying that these rumors are true, but if you look at Nintendo's line up for Quarter 1 and 2 you see 5 games: Endless Ocean, Wii Fit, Mario Sunshine Baseball, Mario Kart Wii, and Brawl.
Endless Ocean is set for January 21. I think that is a lock. Brawl is set for February 10. I think that, too, is a lock. So that leaves Baseball, Fit, and Kart.
I think it'd be safe to say that Baseball comes out in that late April/ Early May slot for the start of American baseball. Which would leave Mario Kart and Wii Fit. There are no games set for March or June. I'm going to say Wii fit comes out in March and Mario Kart gets the big delay and is moved to August. (I think they won't give us a firm date until E3)
Just my analysis.