I'm actually more optimistic about the Wii U than most seem to be here. With the Wii, Nintendo had that infamous inability to meet demand for like a year. So even if the Wii U doesn't have the appeal the Wii had at its launch, it could still sell more in its first holiday if Nintendo can stay on the ball with shipments.
That said, I still don't see it selling more than 3 million, and since the Vita has almost sold that many already, it doesn't look good for this prediction.