With Super Mario 3D Land, Nintendo introduced a firmware update to improve friends lists. Most notably, it added a "Join Game" feature to jump into a multiplayer game, which was an odd choice to include in a strictly single-player game. The first game to actually support the feature is the upcoming Mario Kart 7, so now we can see just how it works.
A short video clip from Tiny Cartridge shows the process of joining up for a game. You need the cart inserted, for obvious reasons, but the game starts up automatically if you hit Join Game. You can't jump into the middle of a race in session, but the feature does let you spectate the match while you wait.
These are baby steps into proper online functionality, but at least Nintendo is giving it more of a try than they have in recent years. Maybe by the time the Wii U releases, they'll have it all figured out, right?