Well, this sucks. ONM (Official Nintendo Magazine) printed a short note about Mario Kart Wii in their latest issue saying it would have 16 player online, stunts, and online battle mode. Well, it seems that was all speculation on their part:
“The details printed in Official Nintendo Magazine 21 regarding Mario Kart Wii were healthy speculation on our part and are in no way an official announcement from Nintendo. We are sorry if we have mislead anyone. We do not set out to mislead our readers. ONM apologises profusely for any misunderstandings that this has caused.” - ONM
So what will it really have? My guess is 12 player online and online battle mode. As for stunts, I don't know about that one. Even so, it'll be a great game.