In 2003, Microsoft deployed Windows SharePoint Services and Microsoft Office
SharePoint Portal Server to create a global enterprise portal and team
collaboration solution that supports more than 60,000 users, 250 group and
division portals, 50,000 team sites, and manages more than 2 terabytes of
information.

For more details, checkout
www.microsoft.com/.../MSWebTWP.mspx.

Here's also a good reference from a Web front-end server performance
perspective:
www.microsoft.com/.../
stsb07.mspx