Among the many of those disturbed by the Licensing of vSphere 5, it seems that OS X 10.6 Server will be supported as a guest operating system.
What's New in VMware vSphere 5
vSphere 5 Pricing
To run Mac OS X in a virtual machine, ESXi must be running on Apple branded hardware.
ESXi 5.0 introduces hardware support for Apple’s Xserver 3,1 server.
Supported versions include Snow Leopard 10.6 through update 10.6.8 with supplemental update v1.1.