CNet has a great Case study on libel law and Web 2.0. You can't hold someone legally accountable for what they say on a wiki.

That's a lesson that's emerging from former journalist John Seigenthaler's run-in with an article in the online, anyone-can-contribute encyclopedia, which for four months carried an article falsely linking him to the assassinations of Robert Kennedy and John F. Kennedy.
But as angry as Seigenthaler was, and as untrue as the article had been, it's unlikely that he has a good court case against Wikipedia, according to legal experts interviewed by CNET News.com. Seigenthaler himself acknowledged as much in a USA Today op-ed piece.

I wonder if that will change with the new editorial quidelines from Wikipedia?