VMware recently moved vSphere and ESXi to Version 5.0. In the list of new features, the virtualized infrastructure will now support Mac OS X Snow Leopard Server.
The release of vSphere 5 introduces support for Apple's Mac OS X Server operating system on ESXi 5.0, with the following support considerations:
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.
In addition, deep on the virtuallyGhetto blog, I was able to find a screenshot of someone running Lion Server.