Wikipedia is without a doubt a cultural phenomenon. I frequently link to it in my blog posts. I suppose I could link to Encarta but in case you hadn't noticed, it's gone
Anyway, since many people link to Wikipedia for the same reason I do, it's not surprising that Wikipedia pages often show up at the top of search results. That's just the way the web works.
Except when it doesn't. Wikipedia marks every link with a "nofollow" attribute instructing search engines that they should not follow the links. That's telling the search engines: "please notice who links to us but please ignore who we link to." When I link to Wikipedia, their ranking on the web goes up as they get some of my link juice. When they link to me with a nofollow attribute, they don't share the juice. This is a classic case of applying a big hammer to a small problem and breaking other things in the process.
You might wonder how this could happen on Wikipedia, which is all about sharing information and consensus, etc. etc. If you read the policy article, you'll see that the change was instituted unilaterally at the request of Jimbo Wales, one of the co-founders of Wikipedia, despite consensus from Wikipedians that this was not the right solution. In case you have doubts, Wikipedia is not a democracy. But I digress.
In order to understand what the right solution is, first we have to understand the problem. The problem is spam links, links added to Wikipedia (and other sites) in order to push the spammers' pages up in search engine rankings. A proper solution would make that infeasible. So all that's necessary is to apply the nofollow attribute for some length of time after the link is added. This gives time for the link to be removed by Wikipedia editors or for the spammer's website to be shut down, rendering the link moot. Obviously, there can be refinement to this idea, but the general concept is sound.
So, c'mon Wikipedia (that is, Jimbo), it's time to start following along with the rest of the web.