There’s a great presentation on Slideshare.net that shares the storied history of Moodle at Open University (@openuniversity), their decision to adopt the open source software, it’s subsequent customizations and upgrades and the particular challenges that one of the worlds largest Moodle has faced. Note that OU has over 500,000 registered users and an estimated peak concurrent user role of over 2000!
Open University has provided support and direct development contributions to many of core features (not to mention re-writes of more functional module such as forums, blogs, a new wiki and the new re-write of the quiz editing system) over the years and continues to be one of the driving forces of innovation to the Moodle platform.
Check out the presentation at this link (or embedded below): http://www.slideshare.net/ram65/moodle-at-the-ou-moodle-moot-2010
There is a lot of great information pertaining to large scale Moodle installations and the various efforts that are required to keep them running at maximum efficiency.