Wikipedia now adds the “nofollow” attribute to all external links. The reason given for this is to battle spam links, like those of blackhat search engine optimizers. The change is already active, as you can see by checking outgoing links on Wikipedia’s articles. (In German Wikipedia, this change was already activated for a longer time.)
Originally, “nofollow” was aimed at comment spam of blogs and such. It then entered the realms of online advertising links. It now covers wikis as well. (The attribute’s name, “nofollow”, is unclear on whether any of these applications are correct; no-follow doesn’t describe what the link is, but rather, what searchbots are supposed to do with it – this is also why we have endless discussions about where and how to implement nofollow.) Back in the beginning, Google – co-inventor of the attribute, along with Yahoo and others – announced that nofollow ought to be used …
… anywhere that users can add links by themselves, including within comments, trackbacks, and referrer lists. Comment areas receive the most attention, but securing every location where someone can add a link is the way to keep spammers at bay.
Wikipedia, Nofollows, Links