I didn't see this posted anywhere, and I can't use the shiny search bar, so....
I saw this over at Penny Arcade, so some of you guys have probably already seen it, but Blizzard helped catch a drug dealer.
Basically, the guy had fled to Canada but continued to play WoW. The authorities chasing after him found out he played WoW, then subpoenaed Blizzard for his information. They complied and gave the authorities all of his information, from it IP address down to the server he played on.
Needless to say, it was easy to find out where he was and have the Canadians arrest and deport him.