A Cre8asite Forums thread reports that Wikipedia has added nofollow attributes to all the links within the English Wikipedia. Joost had the scoop sharing that due to the most recent SEO contest, the founder, Jimbo Wales, has ordered that the nofollow be added back to all links within the "en.wikipedia.org's article namespace."
Wikipedia did have the nofollow attribute on all those links in the past, but removed the nofollow attribute from the Wikipedia in November 2005.
The Cre8asite Forums thread has lots of discussion on if this is a good thing or bad thing. The reason Wikipeda made this move was to discourage "vandalism" but has the nofollow attribute discouraged vandalism in the blogosphere? I honestly can not say it has.
In any event, most folks in the thread feel this is a good move. I think it is wise for Wikipedia to give this a try one more time and see if it works. They can always decide in a few months if they want to keep the nofollow on or remove it. Either way, they will be able to test their theory.