Reinout van Rees blogged about a Plone Conference 2007 presentation given by Jean-Paul Ladage about an intranet built with Plone for the 6,000 person research department of Phillips. Unlike many Plone websites, this project required large scale personalization.
The site has only logged-in users and a few thousand actually log in every day. This is different from most of the Plone sites, where you have a large majority of anonymous users. The users are authenticated against a 200,000 user LDAP database.