I don't think it was a fad, more than was a promise that didn't deliver. The thing that caught the attention the imagination of the world was not graphics or even the games(to some degree), rather it was the sneak peak into the possible future of technology. All those who bought the console were not necessarily gaming affecianados, but were people who dreamed of flying cars, of virutal reality, something out of the Jetsons. Motion controls were the next step into getting a real light saber in real life, that alone garnered the Wii attention. I just don't think it's a fad when there's so much more interest in something that wasn't perfected.
As for the DS, to be honest that seems to be more of a fad than the Wii. Sure, the ds and the 3ds have long legs and they travel far, but other than what they've shown so far, they are soon to be on the down-swing of a technology that will not repeat in the next generation, if consumers are smart and those who are manufacturing them are smart as well. Just imagine a third iteration with the same clamshell two screen combo with nothing more to add than that and process power and 3d. If you think about it, the jump from ds to 3ds was nothing more than a clearer picture, 3d, and more power. Plus, I'm wondering if there weren't friends who told friends that they'll battle them in pokemon or race them in mario kart, that the 3ds would be selling as hotly as they are now. And that is the ultimate sign of a fad when people start buying because everyone else is buying them.