Yesterday Barry blogged about Google testing a search box for site search within general search results. Today the feature is being fully rolled out for selected queries. It’s really an extension and elaboration of Google Sitelinks. The Google Blog explains:
[O]ver the past few days we have been testing, and today we have fully rolled out, a search box that appears within some of the search results themselves. This feature will now occur when we detect a high probability that a user wants more refined search results within a specific site. Like the rest of our snippets, the sites that display the site search box are chosen algorithmically based on metrics that measure how useful the search box is to users.
Here are some examples: