> I wonder if it worked in this case because it started a movement?

It was continuing a movement and giving it visibility outside web developers. The youtube banner happened mid-2009, web developers around the world had soured on IE6 and actively trying to kill it off since 2005~2006.

What the youtube banner provided was a way to finally get the word out, and an argument to give PHBs: it became much easier to sell browser upgrades (or not supporting IE6 anymore) when you could point to Youtube / Google and go "these folks have put their foot down".