We didn't really expect Mario Kart Wii to have much in common with Wii Fit other than sales, but that's the approach Nintendo is taking with the kart racer's online features. Nintendo previously announced that Mario Kart Wii will use a "Mario Kart Channel" that, like the Wii Fit Channel, stays accessible from the Wii Menu even when the disc is not in the drive. You can use this Channel to view and download ghost data of other players and race against them (presumably you'll need the disc for the racing parts). Nintendo has released additional information about this Channel which reveals just how useful and smart their detached Channels can be.
The Channel lets you register friends and check their online status (awesome), look at rankings, and view the current Competition. What's a Competition? "Nintendo will on a regular basis invite players to speedy challenges similar to the Missions of Mario Kart DS." It kind of sounds like Nintendo is actually encouraging online play for once.