Per a directive from Jimbo Wales reported in Wikipedia Talk, all external links in Wikipedia are now "nofollow." This is intended to reduce the volume of spam created by SEOs. The decision was triggered by an SEO contest discussed on the Wikipedia Administrators Noticeboard. As observed and commented on by Andy Beard, the "nofollow" is not universally observed by search engines.
The "nofollow" was developed to counteract blog comment spam where links were not trustworthy. For Wikipedia to render all external links as "nofollow," while an efficient way to cut down deliberate spam, in essence calls into question all of the links in this resource. Are any to be trusted? What about the value of the Wikipedia as a source? This begs for a better solution.
Postscript (Kevin Newcomb):
Most of the Wikipedia editors are coming out in favor of the move, and some are even wishing violence against spammers. Once again, SEOs and others that try to get legitimate backlinks from Wikipedia for relevant sites are being lumped together with spammers who are trying to manipulate Wikipedia results with irrelevant links.
At least one link-building SEO, Rand Fishkin, is in favor of the decision. And at least one Wikipedia editor, BozMos, is predicting the decision will not achieve the intended result of reducing spam, and could even backfire.
A lively discussion is ongoing at the Cre8asite Forums.
Postscript 2 (Kevin, 1/23): Matt Cutts got back from vacation, and after plowing through his pile o' feeds, had this tidbit to say about the Wikipedia "nofollow" decision: "I don't expect this change to affect Google's rankings very much, but it's good to see the Wikipedia folks paying close attention to link spam (and open to refining their trust for external links)."
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