Some guy in the states actually did a while ago. It was on the news.
He was some porn guy (actually credited with starting the voyeur wife and home movie on the internet stuff IIRC) who was really good with computers. He hacked into an Al Qaeda site, took control over it, and was just reading what was going on. He even posted a few fraudulent messages that got the guys to do some things. He told the FBI about it, but they didn't think it was that big of a deal. Al Qaeda finally caught on and I think they shut down the site.