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

That was exactly what I thought when I saw the Wii Us enclosed under the TVs

Just as a side note ...

This could also explain why the third tech demo was shown behind closed doors and was suggested to be significantly better than the other two. Essentially, Nintendo might have had a limited supply of prototype hardware units that were (probably) in cases similar to this, and were showing off the demo to people who would understand that just because it was in a case like this didn't mean that it was going to be released like this; and would also (potentially) forgive a major system crash and allow Nintendo to boot up an alternate system.

When 360 was released, it's GPU was much more current to other GPU's than HD 4890 is now. 360 used at the time highest-end GPU and very few PC's had that good GPU. R500-based GPU's on PC were released at the same time as 360 was released. So, 360 used CURRENT generation at the time and 4890 is two generations behind. 4890 is dirt cheap compared to GPU 360 used in 2005. So why is it hard to believe that Wii U would use 4890?

Because what Nintendo showed not support that afirmation.

Quote me to eat crow .

Isnt it customized and a lot stronger than a R770?

What does this early presentation of Perfect Dark 0 on the XBox 360 tell you about its hardware?

It could very well be, but the hardware shown was way too small. We would be in for RROD part 2 even at 28nm. 

Anyway, maybe it is true not even Nintendo being sure yet of what's going to make into the console.

How do you know? Do you know how many 32nm fabs are actually operating (you qould be surprised) for mass manufacturing? If they really had a 32nm line, wouldn't they shrink the CPU first?

Let's not forget the 4890 is an RV790 chip, not an RV770 chip. Even at 45nm, we are looking at TDPs of >100Watts, and adding the rumour of a superfast CPU with a TDP over 125-150Watts, that would make a hell of a lot of heat to dissipate for such a small WiiU box as shown in pix (I love the picture where a Ninty guy places the WiiU into an almost completely enclosed TV stand.. that unit would melt within minutes..)

Didn't IGN report a demo showed close-doors during E3? If I remember correctly, that demo (Japan Street) was better than the bird demo, even to the point the editor ask himself why he bothered to look the 'hawk' demo.