As Wikipedia's user base grows, the charm of the site for spammers grows. The online encyclopaedia's popularity and its ease of editing made has made it a desirable target for link spammers - successfully placing a link in a Wikipedia article could quickly do good things for a website's standing on Google or Yahoo or MSN. Wikipedia has always depended on a vigilant anti-spam team of volunteers and on blacklisting problem sites. The spam link problem has, however, grown to massive proportions and Wikipedia is considering, to the outrage of certain users, to solve the issue by applying the "nofollow" tag to external links. The rel="nofollow" link tag was introduced in late 2005 by Google as a way to fight comment spam on blogs. It was quickly adopted by the other major search engines and blogging software providers as well.
Most search engines give more weight to websites that are linked to by lots of other sites, the idea being that the number of links is a proxy for the quality of the linked site. The "nofollow" tag tells search engines not to give any weight to that a particular link in calculating a website's ranking. Introducing "nofollow" has consequences, though: it means that legitimate external links don't get the ratings boost and user-contributed links are branded untrustworthy by default. Jimbo Wales, Wikipedia's founder, has asked for implementation of "nofollow" on all external links from encyclopedia pages (user pages will not have the tag applied). In Wikipedia discussion pages, some posters raised questions about the effectiveness of the policy. Although using the tag will discourage some link spammers who only want to boost their search engine ratings, plenty of spammers don't care about search results, they care about traffic.
News source: Ars Technica