Parallels today announced an addition to their MacOSX Server virtualization lineup. Parallels Server 4.0 for Mac Bare Metal Edition allows you to run virtual machines on your XServe or Mac Pro without installing OSX Server on the machine first. Older versions of Parallels Server for Mac ran on top of OSX.
The elimination of OS X as the primary OS cuts un-needed overhead, giving administrators the ability to add more virtual machines, including MacOSX Server.
More information here including a downloadable trial.