Today I was searching Google for the keyword wikipedia. I found something new on the search results. I found a search button text box and button under the search result of wikipedia mentioning to search site wikipedia. I tested it for few popular keywords and for all them it had this search box. From the test I did it is evident that site with more than thousands of articles get this box else just the site links 
Happy Googling 
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April 6th, 2008 at 8:38 am
Nice! However, why were you googling for wikipedia ??
btw, great that you’re using reCAPTCHA .. you’re the first website i’ve seen that is using it