"It's not something you can toggle on and off in ANY game," said Bowling. The special mode can only be played in private matches -- toggled on or off just like "Hardcore" settings -- or in playlists that are designated for third-person play. According to Bowling, everyone playing in a third-person playlist will be restricted to the viewpoint. "This is just another cool way to play the game," he said, "you obviously will still have the option to play gametypes normal as well."
Our friend, the internet, has an inscrutable video of some person running around in third person, but you're probably better off waiting for the "awesome trailer" the folks at Infinity Ward have planned.