According to a news item in the Chronicle of Higher Education's "Wired Campus" Wikipedia has started removing not only content but also the history of that content ever having existed on the site. (Read What Wikipedia deletes, and why).
Two researchers at the University of Pennsylvania have completed a report redacted content during 2010 : What Wikipedia Deletes: Characterizing Dangerous Collaborative Content. (.PDF, 4 p.).
Wikipedia has had to resort to the same human-labor-intensive work that characterizes scholarly publications due to the libelous nature of the offending content, in addition to blatant violations of copyright law.
My favorite comment on the Chron's Wired Campus story: "Plagiarism in Wikipedia - Do not copy and paste stuff into the place that everyone copies and pastes from."