Google Site Seach In Organic Results

I'm wondering if I've been hiding under a rock or just completely missed the whole google "site search box" magically appearing under selected websites "site links" in Google's organic results. At first it feels a bit confusing that there are now 2 search boxes within pixels of each other - but then I start to see this is an ingenious idea - saving users considerable time and effort. But, is it only for affiliated Google sites like Wiki and Dmoz? Because I don't see this same functionality for websites that are used almost exclusively for search like Yellow Pages...?