A lot of people complained that this was a lacklustre start for the system, but they haven't taken into account the economy, how badly the system has been both marketed and advertised, and that it's releasing at a higher price point than the Wii did (which apparently sold 600K in its first week in the US). However, 400K is a pretty respectable number given the real world circumstances. While not every store in the US has sold out, the majority of retailers have, and those still seen in the wild by people are the Basic Pack, meaning the Premium Pack has been the system of choice.
What makes it even more promising is that while the Wii U system is sold at a loss, the moment you buy a game, it turns a profit. This means that the Wii U must be selling at a marginal loss per unit. I'm sure once Nintendo of America release their software sales for that week, you may find that the Wii U as a whole is turning a pretty penny.
If similar figures maintain in Europe come Friday, and Japan the following Saturday, leading into Christmas, there's every chance the Wii U can sell 1.2 to 2 million units just in that timespan alone. Also given certain software in the launch window will entice buyers to invest in a Wii U upon release (such as Rayman Legends), by the end of the fiscal year alone, I can see 2 to 2.5 million units sold, and that's irrespective of the teething problems the Wii U has at the moment. I can definitely see ZombiU being the game of choice for European gamers, and in Japan, that Monster Hunter 3 Ultimate code in the Premium Pack alone will print Nintendo a whole heap of cash!