CiviCRM can work within virtually any content management system (what we call a "user framework"). We have determined it is feasible for CiviCRM to work within Plone, but no one has dedicated the time or money to make it happen.
Besides being able to handle the users (fairly complex), there is the other major requirement of being able to read or modify content stored in CiviCRM from the CMS's interface. Fetching a specific group of contacts and displaying them has been done from within a SPIP CMS generated page.
At the campaining camp in Oxford in March 2010, we worked on this interface with Plone. The architecture involves writing a wrapper in Python that uses CiviCRM's REST API. The wrapper provides an abstraction layer to create, read, update or delete any CiviCRM content. This allows Plone or possibly other Python CMSes (or programs or frameworks written in Python) to interact with CiviCRM's data without having to know that CiviCRM is written in a different language, and possibly running on a different server.