It would definitely have been big and driven hardware sales. Though, to some degree NSMB is a better fit on Wii, the controller suits better and the console itself is already in with a wider audience.
I'll say 15-20 mill, which is much less than it'll do on Wii.
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.