Hello,
I just thought that I would jump in quickly to verify that Moodle is supporting some large installations. I was with the Open Polytechnic of New Zealand when we adopted Moodle as part of the NZOSVLE project, which is probably the installation to which you are referring. The team there feels that there will be no trouble scaling to the UK Open University and beyond. If you are interested in learning a bit more about some of those activities you might want to refer to Eduforge.org where the project is hosted (https://eduforge.org/projects/nzvle/).
Integration, I think, is a more complex topic which depends on what is meant by ?integration?. We have been chatting a bit about integration and interoperability at SUNY (State University of New York). The discussion has resulted in the development of the LMOS (Learning Management Operating System) notion, which is being floated at https://eduforge.org/wiki/wiki/lmos/?pagename=HomePage. If you have been following along with Michael Feldstein?s blog, you probably already have a flavor for this. My point is that at the level of integration needed for the LMOS, we felt that Moodle was not well suited.
Cheers