This is something Naughty Dog achieved by taking a lot of the processes on the main processor and shifting them onto the SPUs (which may be one of the reasons why it could never work on Xbox 360). This allowed the developers to significantly increase the quality of the animations and other aspects of the game, leaving the main processor to "focus on getting all of that richness in the backgrounds." Well, whatever that means, we're glad they did it. This game is purty.
Head past the break to watch the video interview -- around the 3:37 mark is where she starts talking about the powah of the PS3.