Wikipedia is a perfect storm of pagerank so unless google have specifically made an exception for it, links from there must be pretty valuable. IANASEO.
For exactly this reason Wikipedia adds "nofollow" attributes to all external links on Wikipedia. You will get no SEO / page rank link juice from adding links to Wikipedia. You will probably get organic traffic though.
Duh. Can't believe I forgot about that. Not really on the ball today. What am I even doing here? This is meant to be my holiday...
Just because a link is "nofollow" does not mean you get no juice. Being linked to from wikipedia definitely helps a site's rankings...
Argument that "nofollow" = "no seo juice" is debatable. Many SEO believe that lots of links from Wikipedia does help.
I have no data of my own to support the above. Theory says that "nofollow" will not help is SEO.