This is a feature that already existed in the API since quite some time. Traditional Google Reader clients have the ability to "star" articles and the flag was added to FeedHQ when we implemented Google Reader's API. Google reader clients were able to see and set stars but not people who use the web interface.
Starting today this isn't an API-only feature anymore. There is now a "star" / "unstar" action on individual article pages, as well as a new Starred view.