There are some restrictions as you might expect. The following is directly from the vmWare website:
You can create a Mac OS X Server virtual machine, with these restrictions:
- Due to Apple licensing restrictions, you may only create and run this virtual machine on Apple-labeled hardware. For more information, see Apple's Hardware & Software Product Agreements.
Note: The preceding link was correct as of September 27, 2012. If you find the link is broken, provide feedback and a VMware employee will update the link.
- You cannot create a Mac OS X Client virtual machine for OS X 10.6 and earlier. Apple does not allow these Client OSes to be virtualized. With Mac OS X 10.7 (Lion) onwards, Apple has changed their licensing agreement in regards to virtualization. For more information on creating a Mac OS X 10.7 (Lion) virtual machine, see Installing Mac OS X 10.7 (Lion) as a virtual machine in Fusion 4 (2005334). For more information on creating a Mac OS X 10.8 (Mountain Lion) virtual machine, see Installing Mountain Lion (OS X 10.8) as a guest operating system in VMware Fusion 5 (2033778)
- You can only create a virtual machine using a retail Mac OS X Server disk or image. Virtual machine creation fails when using a disk that came with an Xserve or a Mac Mini Server. Those disks are hardware-specific, and can only be used to install the OS on the physical hardware with which they were shipped. They cannot be used on virtualized hardware.For more information, see Installing Mac OS X Server on a virtual machine fails with the error: Mac OS X can't be installed on this computer (1032864).
For more information, see Creating a Mac OS X Server Virtual Machine in VMware Fusion in the VMware Fusion Help.
I have both Parallels Desktop and VMware Fusion on my Mac. I'll have to try this out sometime and install Mac OS X Server 10.8 in a VM.
You can read more on the VMware website.