I found this article http://columbiamissourian.com/news/story.php?ID=21322 about a fellow who was arrested on charges of possesion of child porn because iTunes sharing had been turned on. An odd case, but certainly interesting in that this feature helped bring someone to justice.
it sure turned out to be a good thing in this case. I think the police would be pleased that they caught this sicko... (and in some cases, almost saddened of what this guy did.)
A friend of mine in Europe found someone in a youth hostel sharing similar material using iTunes. Hostel management called the cops and had him dragged away. He was already wanted for some other charges.