Julie Castillo and Cassini project scientist Dennis Matson (NASA/Jet Propulsion Laboratory) say that the moon rotated very fast in its youth and it bulged out accordingly. Its shape was "frozen in time" as the body cooled. They go on to say that radioactive isotopes imply that Iapetus's age is 4.564 billion years old.
No doubt this paper will keep the Iapetus community thinking until September 10th. That's when Cassini will make its closest flyby of the moon yet just 1,000 kilometers (620 miles) above the surface.