All links on Wikipedia now automatically use the "nofollow" attribute, which means that when Google crawls the site, none of the links it comes across get any PageRank from appearing on Wikipedia. SEO contest concerns aside, this also has the effect of consolidating Wikipedia's power. Now it gets all the Google juice and doesn't pass any of it along to the sources from which it gets information. Links are currency on the web and Wikipedia just stopped paying it forward, so to speak.
It's also unclear how effective nofollow is in curbing spam. It's too hard for spammers to filter out which sites use nofollow and which do not and much easier & cheaper just to spam everyone and everywhere. Plus there's a not-insignificant echo effect of links in Wikipedia articles getting posted elsewhere so the effort is still worth it for spammers.