This game has just, like, everything going for it.

Mario is insanely popular. 2D ones even more. Wii is really popular. Wii also has a diverse audience, and a 2D Mario is like THE bridge title that can appeal both younger and older, male and female wii owners, and "hardcore" and "casual", and this one will have harder levels and superguide, which furthers the point.

Then, this game is shaping to be a "best of Mario", taking every element everyone loves about Mario and cramming it in there. And there's multiplayer to boot.

I think Nintendo's estimates are quite realistic, and this game might actually sell better then the DS one at the end of the day.

considering wii play is the best selling title, it sucks; nsmbwii will be awsome, as the other guy said this is a bridge, it can easily outsell wii play