Version 10 works on RHEL5 with a few modifications and installing the perl modules by hand
by editing /etc/redhat-release
I'm already working on the new project - cpanel split over 8 servers
2 x web
2 x mail
1 x cpanel
1 x mysql
2 x clustered windows nfs servers with a san backend
All modularised so we can add extra web/mail servers depending on load