Update:

> there's an interesting tweet by IBM from June 2011 which seems to confirm that
> the Wii U actually uses the very same POWER7 *chip* (not just the same core) as
> IBM's Watson computer, which would mean an eight-core POWER7 chip. I find that
> very hard to believe.

If the recent information about Wii U's CPU being a three-core chip is true, this would mean that IBM purposely spread false information in June 2011. There's not even a POWER7 chip with 3 cores* (4 cores being the smallest variant).
And from last month two other tweets from the same IBM twitter account that, citing an old Engadget article, suddenly read way more vague and contradict the older tweet (same chip as in Watson vs. custom chip):

http://twitter.com/IBMWatson/status/231902074890248192
http://twitter.com/IBMWatson/status/240241146213842944


* Edit:
In http://www.redbooks.ibm.com/redbooks/pdfs/sg248003.pdf (pages 20 ("6"), 22 ("8") and 27 ("13")) it is revealed that "C1" is IBM's name for the CPU cores in the POWER7 chip.

[ Edited by Andreas_Wolf 07.01.2013 - 17:42 ]