You just saw that one? That's a pretty old one now, but still one of the better ones I've seen. There's two more hosted by T-Mobile in Europe that are very popular. I recall Purdue Univ having a flash mob event, as well as several other college campuses. There's also some that were organized by a smaller group, but were hilarious (Check out one titled Look Up More).
I love these. With all the hate and negativity in the world, stuff like this reminds me there is hope for my daughter and her kids, and their kids, and their kids.