With the limited information we have been given and the rumors about what components that are supposedly in the Wii U, I think it is fairly safe to assume that the Wii U probably has between 4 and 8 times the theoretical processing power of the current HD consoles. On top of that, it would not be surprising to find out that Nintendo is using heavily modified components that are more efficient and/or easier to develop for resulting in comparatively higher performance than you’d expect from hardware with similar theoretical performance.
While I have no doubts that Sony and Microsoft could easily surpass this performance range (especially if they launch a system a year or more later and a $100 or more expensive), with the added cost and diminishing returns of improving graphics in games, I’m not certain that the increased performance would be used enough or be meaningful enough to make someone choose these consoles over the Wii U.
To use an example from PC hardware, while there is no doubt that a Radeon HD 6870 is significantly more powerful than the Radeon HD 5770 the added cost and limited real-world benefit make it difficult to recommend to anyone to buy the higher end card.