I don't know but the Parallels Desktop 4 user guide says Mac OS X Leopard Server 10.5.x http://download.parallels.com/desktop/v4/docs/en/Parallels_Desktop_Users_Guide/23583.htm Later versions of Parallels Desktop also list Mac OS X Snow Leopard Server 10.6.x
Hmmm.... It seems odd that they would mention Snow Leopard Server but not Snow Leopard unless Snow Leopard is actually not supported.
Apple's EULA doesn't allow running Mac OS X client under virtualization so Parallels doesn't support it. I don't know if client or server works. I'm just saying what's in the documentation. If you have hard disk space, a Snow Leopard disk, and some time then have fun trying it out.
Virtualizing Snow Leopard Client it is prohibited by Apple EULAs, so it is prohibited in Parallels Desktop. Attempt to install will cause a message saying that this guest OS is not supported and VM stop.
Note that the Startup Disk preferences panel in System Preferences can tell you if a system is "Mac OS X" or "Mac OS X Server". It gets this information (including the OS version number) from a plist in the /System/Library/CoreServices folder. Only Mac OS X Server disks or installer disks have a file called ServerVersion.plist which is identical to the SystemVersion.plist file in the same folder but the ProductName is "Mac OS X Server".
