The first data are already out about the collision between the Deep Impact probe and the comet Tempel-1 last week. The probe made a crater about 300 feet wide and of unknown depth.
Comets had been proposed to be "dirty snowballs," with water holding together a grab-bag of other materials. This has since been modified to icy dirtballs, with other materials predominating. Researchers were nevertheless suprised to see very little water in the fresh ejecta. Instead, the bulk of the material kicked out of the comet was very small-grained, on the order of talcum powder, suggesting that short-period comets might be more like interplanetary dust bunnies, stable only in the deep vacuum and low gravity of space. One interpretation that needs to be modelled out is that dessication from solar warming extended deeper into the comet than expected.
Lots and lots of observatories were watching the event, so it will still be some time before a final run-down of the insides of comets is available. Tempel-1 will return in 5 years and it may be possible to continue observatoins then.