Parallels has announced, a beta version of their server software which is capable of booting Mac OS X 10.5 Server, this makes it the only virtualization suite thus far to take advantage of Apples "amended" EULA which allows OS X 10.5 Server to be run inside of a VM.
How well this will perform is up for debate, you would assume that Parallels would produce some tools in order to optimize the OS like they have for Windows, Linux,etc.
The question about this virtualization leaving the door open as it were for the emulation of OS X 10.5 on generic PC's using Parallels workstation, Director of Corporate Communications told macrumors.com that they have not enabled this functionality in the Windows and Linux versions of their product as this would violate Apple's EULA for Mac OS X, and Parallels wished to continue their good relationship with Apple.