Site Search Box In Google Search Results *?*

Ok, so sometimes when i search for a site in google it comes up with another separate search box in the results. This search box gives you the option to search that particular site.

Ive taken a screenshot and highlighted the part im talking about...i may have gone a bit to artsy in photoshop


Does anybody have any idea as to how i could do this for my site?
Is it even possible or have these sites got some sorta deal with Googe?

 

I think it's exclusively for large authorized site...

 

yup, i think your right as both those sites are like in the top 5 visited sites on the net.