Internally, Microsoft has a similar situation with several business divisions as well as hundreds of subsidiary offices around the world. They're currently managing several hundred portal sites and 10,000s of team sites.
For more details, checkout the Microsoft IT Showcase white paper "Microsoft Web Enterprise Portal" (www.microsoft.com/.../MSWebTWP.mspx) as well as the related "Deploying SharePoint Portal Servers 2003 Shared Services at Microsoft" MS IT Showcase white paper (www.microsoft.com/.../sharetwp.mspx).
If you have any questions regarding these, email me. As well, I'm going to be presenting the Shared Services talk at the Microsoft TechEd 2003 conference on Thursday afternoon, May 27th, in San Diego and discussing Microsoft Web as a specific case study.