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.
I haven't spoken with Jimmy about this, but I think this is the wrong decision. As blogged previously, nofollow is a partial fit for blog comments, but not wikis:
I should mention that the wiki world isn't wild about nofollow for at least one simple reason. On a blog you have an author and the audience (commentators?). Within a wiki, everyone is an author. We are still evaluating where we will use nofollow, I personally see it as great industry cooperation creating a tool to use.
Unfortunately this is throwing the baby out with the spamwater. We need an alternative to this broad stroke. Wikipedia is perhaps the best source of outbound links on the web, with a no-spam policy that while expensive to implement, largely works. I'm concerned about how this effects the health of the web, search result and what precedent it sets for indexing public wikis as a whole.