As was mentioned in starmen.net's
Wii topic, a search of
USPTO's trademarks reveals that Nintendo has not trademarked the name. Could they have just not trademarked it yet? That would be strange. Of course, Nintendo may just be waiting so the name didn't leak before they posted their marketing materials. Or maybe they just assumed no one would want to steal the name?
The other possibility is that Nintendo does not plan to actually use Wii, which would also not make much sense after having made such a big deal out of it.
(I apologize if this was mentioned somewhere on this forum before, but I do not remember seeing it and search turns up nothing.)