Two studies to warm libertarian hearts:
David Friedman reports on a study showing that increased availability of pornography (specifically via the internet) decreases the incidence of rape.
And Tyler Cower links to a study showing that violent movies decrease the incidence of violent crime.
Mind you, I haven't read either study, so I can't vouch for their reliability. But the basic arguments make sense: Virtual sex is a substitute for real sex; virtual mayhem is a substitute for real mayhem.