I just noticied that Wikipedia is now adding the rel=nofollow link to external links. I think its about time. I can't imagine the headache that the editors go through and the endless spam that they have to deal with. I saw this day coming a long time ago, because of the authority status that Wikipedia has and the fact that it is in the top 5 for many Google searches, every SEO knows a incoming link from a Wikipedia article is like gold and many even go as far as to add their site's links to as many articles in as many categories as they possibly can.
Its seems like the transformation is not complete as some articles are using the tag and some aren't.
How did I find out? I visited the site today and I use the SEO for Firefox plugin which shows links that have the rel=nofollow tag in bright red.