Wikipedia has decided to make the external links in the encyclopedia "nofollow" links.

At Jimbo Wales' directive, all external links within the English language Wikipedia are now coded "nofollow" — this should help cut spamming immensely once word gets out in the SEO community.

This was mentioned in the discussion Wikipedia:Administrators' noticeboard#Globalwarming awareness2007/SEO world championship — expect a spam onslaught.

This is done to contain spam. But what the real impact will be only time will tell. But if past instances are something to go by, spammers will find alternate and probably, more harmful ways of getting incoming links.