Specialized search engines that focus on a subset of the web are becoming increasingly popular. These services can reduce problems with ambiguous terms (such as the word "pool" that could reference a parlor game, something that you swim in, a group of persistent connections to a database...) by focusing on a subset of the web. Google is enabling this specialization through its Co-Op technology. At least that is what Ric Shreves from Mambo specialist Water and Stone (they also work in WordPress, Drupal and Joomla!) is using it for. Water and Stone has recently launched CMS Search, which indexes around 75 open source CMS project sites.
This looks like an interesting tool. There is a "refine" feature that limits search to a subset of sites. Try using this to search for a feature and then drill down to see what the community or the documentation of a project has to say about that feature. Unfortunately, this refining feature only works with projects that Water and Stone specializes in.