The problem with VMWare Server is that it is a discontinued product for longer time and they don't provide us with a suitable replacement.
So our institute had to switch to VMWare workstation that can be run as a server, too. I'm running CentOS 5.5 as the Host OS and 5.5 and RHEL6beta as guests. We didn't try out RHEL6 (waiting for CentOS6 :-)
GS