Given the Wii-U's size, it's unlikely that the R700 variant in it would be derivated from the HD4770, 4850 or 4870.
Would itmagine that it uses a souped up version of the 4600 series, maybe running anywhere between 320-480 Stream Processors like the mainstream GPUs found inside laptops today.
That should allow the console to game at 720p at relative ease, and allow games with simpler graphics to be rendered at 1080p easily I would imagine. The power of 320 Stream Processors and Shader tech implementation should allow the Wii-U to surpass the GPUs in the 360 and PS3 with relative ease.
Not convinced about the controller though, doesn't look as if it'll be comfortable.
If its heavy though, we may end up with more casual gamers with better looking, toned arms and chests? *cheeky grin*
Looking forward to the next Smash Bros game =)