The Wii is a fad. And so were pet rocks, Cabbage Patch dolls, hoola hoops, hoppity hops, etc. Here's to those who make a bazillion dollars selling "fad" products.
The challenge for Nintendo is creating the next "fad" after the Wii.
EDIT: Here is the definition of "fad" from dictionary.com:
"a temporary fashion, notion, manner of conduct, etc., esp. one followed enthusiastically by a group."
I guess the question here is, "how long is temporary?" My guess is that the Wii will remain a fad until Nintendo invents another groundbreaking console.