Google added a new feature to its core search with a new search box along side results to search for content with a site.

An excerpt from Search Engine Land

When you type a keyword into the search box and click search, you are essentially searching the site via a site command using Google's search engine. For example, I typed in mortgage rate into the search box within my ny times search and Google returns pages from the New York Times that match that query.

The feature would come in handy for searchers who tend to search for a site and then search within the site for content. It seems like a Google custom search plugin on a Google search page. As the engine that indexes content all over the web, it makes sense for the engine to want users to search within the site using Google only.

Also, the service roll out does not occur for all results. As of now you don't see the box for all sites and it seems this is an approach that will become a consistent feature soon.