Yale University uses CAS as its preferred method for all web-based authentication. Kerberos is the backing authentication mechanism. In the case of a CAS server outage, a hot backup is ready to go in another machine room on campus. Failover is accomplished simply by bringing this machine up with the same IP address as the main CAS server. More than ten thousand users; millions of tickets vended. We run a second CAS server instance for applicants, with its own userid namespace, backed by a different Kerberos realm.
We use Proxy CAS, primarily for our uPortal instance, YaleInfo.
Server specs:
Sun Enterprise 280R
1 x 750MHz Ultra Sparc III
512 MB RAM
Solaris 8