Google Labs is now offering something they call a "site-flavored" search. It is a search that is biased or filtered to specific subject categories. For example, you can specify that any searches done through a Google search box that you put on your site should be bias toward pages that are in the "Computer/Internet" category.
It appears to be an application of the same technology used for Google's personalized search. In fact, the HTML code you use to put the site-flavored search box on your site contains a reference Kaltix in an image called google_kaltix_site_flavored_searchbox.gif. Google acquired Kaltix year ago. The technology behind Google's personalized search was developed by Kaltix. A paper published by some of the people who founded Kaltix gives the details of their personalization algorithm.
Update: An excellent ResearchBuzz article covers Google's new feature in detail.