I guess it depends on how you interpret the phrase "next generation". If you believe that "next generation" is the equivalent of "state-of-the-art", then no, the Wii isn't a "next generation " console because it's lacking state-of-the-art technology. However, if you approach the phrase "next generation" from an evolutionary viewpoint, then the Wii can be considered even more "next generation" than any other console since it has changed the most from its predecessor.
I personally believe that it's best to include both of these interpretations when defining "next generation" which is why I believe that the 360, the PS3, and the Wii are all next generation (or should it be "current generation"?) consoles.