There are quite a few images from the recent Iapetus flyby available now, and they are stunning. Emily Lakdawala has a great collection at the Planetary Society weblog including this mosaic of the satellite as Cassini moved away from it.
The bright areas are thought to be ice common in the Saturnian system moons, like Enceladus for example. The dark areas are ice covered with some dark deposits. More detailed images show the dark material lying in craters and channels etc on the surface.