Miyamoto mentioned something about not wanting to be confined to the television set in an interview a while back, and when asked if Nintendo had something up its sleeve, he only gave the cryptic comment: "I've already said too much".
If Wii is not next-gen, then maybe Nintendo plans to skip a generation in 5 years. (That will be 2011, one year after all the good stuff is supposed to happen [I say that with tongue in cheek because so many official predictions have said in the last ten years that we'll have hydrogen cars by 2010, among many other innovations becoming mainstream, like solar power and all that])