We saw the posts on Intelligent Positioning and Search Engine Watch earlier this month and did the exact same study but with a larger (2955 SERPs), more diverse (1-9 words in length) set of keywords. We used keywords serviced for real clients, which provide a sample representative of more competitive, valuable, and realistic SERPs.
Data for our Sample:
Wikipedia appeared in positions 1-10 for 28.2% of keywords.
*7-, 8-, and 9-word phrases were left out due to low sample size
Conclusion: Sample selection matters. There is no doubt that Wikipedia owns a large portion of search results. But, for competitive phrases, Google does not just give a top 10 spot to the free encyclopedia.