Called "Harmony," the big idea was to make a colorful, Wizard of Oz-type tableau made completely from costumed people. Water, trees, clouds, mountains, butterflies -- each natural element required a cast of thousands in carefully orchestrated maneuvers.
The production company behind the film was New Zealand-based The Sweet Shop, famous for its lavish production quality. To answer your questions about how in the world they did it, a short "making of" video was released:
To learn more about the new 2010 Prius, read Jim's review on MNN.