Well, I never saw the pigs literally cleaning the cage. But both Grizzly and Bean can be very organized. They like to keep their pellet bowls and hay racks in the same spot, and they tend to pile the "dirt" up in the corner.
And yet somehow the cage always ends up messy. I think they run around and spread it accidentally. And they like to drag the hay around so it ends up an even carpet by the end of the day.