Chad will probably speak more eloquently about this later, but as I understand it, the First Amendment in the US recognizes both free speech (the right to write or say anything) and free expression (the right to produce art or images or whatever).

The US Supreme Court ruled on the issue of obscenity as it relates to pornography some 20+ years ago, and ruled that obscene images, speech, acts, etc are not protected by the First Amendment (nor is certain speech, such as yelling "fire" in a crowded theater) The Supremes came up with a set of rules that had to be met in order for content to be considered obscene, and that became what is now known as the three-pronged Miller Test, which I won't reprint here but will provide this Wikipedia link.

Hope that helps.