Just looked at Mario Kart site on Nintendo UK and it says:
Here is the site if you want to check it out yourself. It is in the third paragraph.Similar to Mario Kart DS, players will be able to take full advantage of Nintendo Wi-Fi Connection. Youíll be able to race against up to 11 other players from around the world in massive online races or race against your friends. You can create rooms for friends to join and even text chat while youíre waiting for other racers. The online function of Mario Kart Wii also lets you join a friendís race in action and be a spectator until the next course launches.