Read my answer to @Play4Fun.
Did you see the Zelda HD Demo for Wii U? There are no way that Demo is running in a HD 48xx... that's my point... what Nintendo showed until now is not running in a powerful high-end HD 4890... it's more like a HD 46xx or less.
Other point is the developers talking about the power of Wii U... none said anything to support that RV770 based GPU.
But of course I can be wrong... like that rumor from Engadget.
There is no way we could know what hardware the Demo was running on simply by watching the demo, and even the hardware Nintendo brought to E3 may not be representitive of what the system's true capabilities are ...
Hypothetically speaking, Nintendo could have decided that they wanted the hardware on display to be in a case that will (more or less) be representitive of what the system will be at release. Nintendo doesn't have finalized hardware, and even if they did the hardware they have isn't being manufactured using the correct manufacturing process and wil run too hot for the case they are using, so they use the stock GPU and CPU their system is based on and have it underclocked to keep energy consumption down.
You may suggest that this hypothetical is unrealistic, but I would suggest you look into the hardware XBox 360 games were running on at E3 2005