Dirk Riehle, one of the organizers of WIKISYM ( http://www.wikisym.org/ ) and a researcher for SAP has a great paper on how companies create, distribute and make money out of Open-Source software. It describes XWiki's model fairly exactly, apart from the fact that since we chose to use only the LGPL license for our product we won't be pushing for a double-licensing model anytime soon.
You can find out the full paper at: http://dirkriehle.com/publications/2009/the-commercial-open-source-business-model/ . It's well worth a read !!