Well, reports are now saying that the Wii U development kits that were received by third party companies were actually "underclocked." Essentially this means that they are working on underpowered technology at the moment. Third parties even had some footage to show at E3, but Nintendo stepped in last minute and said that what they currently had wasn't much stronger than graphics offered by the PS3/360, so Nintendo used PS3/360 footage instead.
However, the bright side of this is that if what devs are working on now is underclocked, and that it's possible to render graphics on par with PS3/360, then we can be certain Wii U will be more powerful than both consoles. Having this power will hopefully not put Nintendo far behind the next-gen.