Alright, so looking at things back in later 06, even though the Wii U is set to fall behind the 360 this month provided serious undertracking isn't found to be the case for April, I think the Wii U will actually be able to re-surpass the 360 aligned at the end of the year with absolute ease.
I can see 12 - 14 million Wii Us by Jan 1st 2014 compared to the 360's 8.1 million and PS3's 9.5 million along rougly the same time frame.
I know 14 million in particular seems a little high right now, but the software will be there in a much bigger and significant fashion than it was for the other two, and the inevitable drop to $249.99 will just grease things even further.
Edit: Important to point out that the 360 was only at 4.8 when that first October started, 5.1 for PS3.
Even granting the Wii U only sells, say, 150k per month till October (I could see an average as high as 225k thanks to Pikmin 3), it'll still only be behind the 360/PS3 in the same time frame by about a million, with an all but guaranteed far better holiday season due to Mario Kart (30 million on Wii) and 3D Mario megahitters, steep price cut, and pent up younger player demand (which Nintendo wins in) due to younger gamers not being able to afford a console themselves, thus having to wait till christmas to ask their parents.
Essay Pro