The final Wii U dev kits are out to Third Party developers but with an important revision. When the Wii U Alpha kits were first sent out during and before E3 of last year, the GPU inside them was based on an AMD HD 4770-4850 and had to be underclocked because of overheating.
It seems that Nintendo has given the final dev kits more power and a different architecture to prevent overheating. This comes from a trusted source who has been posting information on NeoGAF and B3D for the past several months. Others have confirmed that this is true as well. Here is the quote:
"Final devkits are out, and rumor has it they differ quite a bit from earlier kits. Supposedly more powerful than anticipated, too."
The GPU in the Alpha dev kits was already at least 2x the power of the Xbox 360 even while being underclocked. It will be nice to see some games running on the Wii U now that the final specs are ready to go. A full powered GPU to go along with less heat consumption to me only means that they shrunk the die and allowed for more processing power and thus could be based on anything from an AMD HD 4850 to somewhere in the HD 5000 series (just speculation of course). I'm excited to see more!