Yes, it's a fad. Wii was designed to be a fad for casual game players and it succeeded.
Honestly I think Nintendo Wii is likely to sell the most consoles this entire generation.
Due to the fact that it's a successful fad, and cheap, compared to Xbox360 and PS3.
I also honestly believe Nintendo is selling Wiis for far more than they're worth.
I bet they could be sold for $99.99 without a loss to Nintendo.