Slate's running an interesting series by Tim Wu on American laws that the government does not enforce. So far they've dealt with prescription drugs, copyright, and porn. I thought the following was worth noting:

What all of these changes reflect are several major shifts in how the U.S. legal system views depictions of sex. The first reveal an acceptance of the libertarian idea that private consumption of nearly any material is not a public harm. That view excepts children and animals as victims, but not consenting women and men who have sex before cameras. In that view, the U.S. legal system has effectively and informally reached the same conclusion as the 1970 commission: Whether you like it or not, private consumption of pornography is just not harmful enough to merit public enforcement.

Yet at the same time, the United States has concluded that it will not be a place, like Europe, where bared breasts grace bus-stop billboards or soft-porn films can be found on regular late-night television. Americans love zoning—compartmentalizing behavior to designated times or places. It's how a diverse nation manages to live together. And so our obscenity system—much of which takes the legal form of an outright ban—is often in practice being used to move erotic content away from public places.