Apparantly because of all the time spent stopping link spammers, Wikipedia is using nofollow tags across the whole site...
http://www.networkworld.com/community/?q=node/10642
Interesting quote:
What happens as a consequence, in my opinion, is that Wikipedia gets valuable backlinks from all over the web, in huge quantity, and of huge importance ? normal links, not "nofollow" links; this is what makes Wikipedia rank so well ? but as of now, they're not giving any of this back.