I'm not sure how solid it is either, but for text there are at least some more constraints on what Congress could ban. Fanny Hill (1748) is a very explicit novel about the sexual exploits of a 15-year-old girl, and despite being frequently banned, it was ruled protected by the First Amendment in the U.S. in 1966: http://en.wikipedia.org/wiki/Memoirs_v._Massachusetts

The decision claims that the it ultimately came down to how the book was marketed. Shaky legal ground.