According to the dutch website Insidegamer.nl this was mentioned by a ubisoft employee during an AC3 gameplay demonstration.
Loose but optimised google translation:
Source:
http://www.insidegamer.nl/nieuws/98237/ac3-cpu-wii-u-net-zo-krachtig-als-ps3-x360-gpu-is-sterker
Loose but optimised google translation:
News | June 8, 2012 - The central processing unit (CPU) of the U Wii is as powerful as the PlayStation 3 and Xbox 360, but the graphics processing unit (GPU) is one and a half times as strong. That is what an employee of Ubisoft stated to Insidegamer.
Ubisoft provided this information from a gameplay demo of Assassin's Creed 3. The editor of Insidegamer noting however that the game is on Wii U currently less good looking than the other consoles, with less detail and less constant framerate.
This can still be solved; because the Wii U is a new console, developers are still getting used to develop for the device. According to Ubisoft Assassin's Creed for Wii U will, at the time of release, be of the same quality as the PS3 and Xbox 360 versions.
Source:
http://www.insidegamer.nl/nieuws/98237/ac3-cpu-wii-u-net-zo-krachtig-als-ps3-x360-gpu-is-sterker