Even the Gamepube got ~20 million units, so I expect Wii-U will do that, minimum. But I don't expect the final number to be "that" much higher, probably ~30-35 million at the end of the day.
Even if you absolutely despised the Wii, it was pretty clear that it had a metric fuckton of hype surrounding it, and one could see the possible appeal it may have to casual players. Wii-U has neither of those things, however. I mean, it's almost impressive at how mediocre this thing is.