If you count the number of pages linking to a Wikipedia page, there may be none but it still appears on the first page of the search results.
Google seems to rank pages from Wikipedia higher up in the results. One possibility is that it doesn't seem to be using the regular PageRank algorithm for some websites. Either that or the PageRank algorithm uses a list of 'favorite domains' when ranking pages. I considered the possibility of 'favorite domains' a little more and Googled it a bit when I came across an article that mentions about Google having access to the domain registration information so I'm guessing that there is a possibility Google checks the age of the wikipedia.org domain registration and factors that in too.