I think a lot of it is going to depend on how third party developers support the console. Right now its more or less staying afloat on the promise and strength of Nintendo games, but at some point it has to be able to attract outside developers as well. If it can do that, I can see it selling at least 50 million units, possibly a lot more. If not, then 30 million at best.
Hey, that's the reason I bought a Wii U!