The website GamesIndustry has revealed that "according to developers familiar with the hardware who spoke on a condition of anonymity to GamesIndustry International" the Wii U is "not up to the same level as the PS3 or the 360," and another developer said "Yeah, that's true. It doesn't produce graphics as well as the PS3 or the 360, there aren't as many shaders, it's not as capable. Sure, some things are better, mostly as a result of it being a more modern design. But overall the Wii U just can't quite keep up." For some reason most of the media is acting like this is 100% confirmed, while this is still just a rumor, and all other rumors are pointing towards the Wii U being more powerful than the Xbox 360 and the PS3.
In order to determine if this is true or a fake, let's look at what Nintendo, AMD (they're supplying the GPU), and IBM (they're supplying the processor) have said about the power of the Wii U.
So far Nintendo has pretty much just said (in terms of power) that the Wii U will have native support of 1080p high-definition graphics, the Xbox 360 didn't have native support of 1080p until 2007. The PS3 does support native 1080p for Blu-ray and the home menu, but it's not powerful enough to support native 1080p for most games.
AMD has said that the Wii U's GPU is based on the R770 architect, which is 3 years newer than both the R520 architect the Xbox 360's GPU is based on and the GeForce 7800 architect the PS3's GPU is based on (usually the newer the tech the more powerful it is). For more information on the Wii U's GPU and how powerful it is see my earlier artilce Wii U Q&A: The Wii U to support 2560x1600 resolution?
IBM has said that the Wii U's CPU is based on "the same processor technology found in Watson" (in case you don't know Watson is the supercomputer that was on Jeopardy), the POWER7 processor, which is a lot more powerful then the Xenon (based on PowerPC architect) processor found in the Xbox 360 and the Cell processor found in the PS3.
So as you see these "sources" are most likely fake and the Wii U will be the most powerful console on the market until the PS4 and the Xbox 720 come out.