Wikipedia is viewed as an online "authority" when it comes to a lot of keywords, and if you are a search marketer (in-house or consultant) you should figure into your equation that they are your competition. For natural search or search engine optimization, more often than not Wikipedia.com is going to come up within the top 10 of the search listings.
I do not make that claim lightly, and not merely from my own experience, there's been a study done by Jure Cuhalev. Jure research Wikipedia's ability to have 1000 random Wikipedia articles rank within the top 10 listings of Google, Yahoo, MSN, and Yahoo. (Hat tip to Steve Rubel for leading me to this)
According to this study, Wikipedia appears in the top 10 of rankings in Google the most. MSN seems to be the least affected by it (however, in light of their recent tweaks this could change). The study did not take Ask.com into account, but I would believe that they are probably even less affected by "The Wikipedia Factor" than Google, Yahoo and MSN because they do not take link popularity as a heavy influencer with their algorithm.
I don't have many qualms with the search engines and how they figure relevancy and authority. However, as a search marketer, I find how much authority Google and Yahoo! give Wikipedia somewhat frustrating. Here's why - it's not a true encyclopedia, it's not a dictionary, it's certainly not a text book, it's a community contributed wiki that has pages that are highly optimized with on-page tagging chalk full of unsubstantiated content.
I link to Wikipedia plenty, because most of the time, the information I see is correct, but that's from my own perception. Those pages could be completely full of inaccuracies and totally false content, and the search engines still perceive that source as "the authority". Within the last few days, Wikipedia has been found to be plagiarizing content from other sources. Daniel Brandt, one of Wikipedia's most vocal critics had been dismissed a merely a whiner, but this time, he's got the proof to back up his claims and the traditional media is taking notice.
The Wikipedia Factor for now won't be going away, and as an organic search optimizer you have to factor that into your strategies and make your clients (internal or external) aware of it and how it will affect your ability to rank within the search engines. I just wish the search engines would view a true encyclopedia (heck they even have a blog!) with more authority than a community contributed wiki.
I leave you with one last parting thought - what happens when Wikipedia figures out a way to monetize itself?