Found on the Planetary Society Blog:
So far, it has generally been assumed that all comets originate from the other two known reservoir regions: the Oort cloud and Kuiper belt. The main asteroid belt is home to, well, asteroids, not comets. But Hsieh and Jewitt have proven that there are comets in the main asteroid belt, and that they almost certainly formed there.
Pretty interesting given that the current assumption is that all water on Earth arrived via cometary bombardment early on in the life of the solar system.