You clearly have never seen the OTRS queue if you think Seigenthaler is the closest to a suit. There have been over a handful of cases (against the WMF or Wikipedia editors themselves) and some even received media coverage, though Bauer [1] is the one that comes to mind at the moment but it certainly isn't the only one. [2]
[1] http://arstechnica.com/tech-policy/news/2008/05/dumb-idea-su...
[2] http://www.citmedialaw.org/threats/american-academy-anti-agi...
The closest it's ever come was this incident:
http://en.wikipedia.org/wiki/Wikipedia_biography_controversy
where some guy's article stated for years that he was a suspect in the Kennedy assassination. He never tried to sue wikipedia, though he did complain a lot.
Incidentally, though, this shows another reason why wikipedia articles on borderline-notable people are bad: the fewer people look at an article the more likely it is that crazy non-facts will persist in the article.