Pluto is probably rocky, probably everything (even comets) are rocky, but have more or less gas either liquified or frozen; Jupiter, Saturn, Uranus, and Neptune are believed to have rocky cores, Earth-sized or better.
Yeah. I've heard astronomers also say comets are giant snow balls. However, NASA's has taken some close ups of some famous ones and they look to have a silicate nucleus.
I agree that Pluto is probably rocky just like all the moons out there. I even think the gas giants have a lot of rock (certainly more than the Earth) but that they just have so much gas that the mass of the gas causes gravity to keep the rock liquid. The gas giants most certainly have HUGE solid cores. The magnetosphere of Jupiter is WICKEDLY powerful and I doubt humans will ever be able to venture anywhere near it because of the ions.
How cool would it be (literally) to stand on Pluto though. First off, it would be pitch black except for the stars. No day whatsoever. The sun would simply appear to be the brightest star in the sky. You would have to bring lots of floodlights or night vision helmets. With night vision you would be able to see Charon, Pluto's moon easily. If you were on the Charon side it would never move.
Just this year we launched a probe to Pluto. It will get there in 10 years. Imagine being on that little world...ten full years away from Earth. You better have something faster.