Apple EULA only allows Server OS to be virtualized
Apple's license agreement doesn't permit the desktop versions of their OS to be run in virtualization, so all of the virtualization vendors including Parallels explicitly disable it from working.
In fact, I believe Apple prohibited virtualization for ANY of their products until recently, when they began allowing the server versions of their OS to be virtualized.
So, you could run Leopard Server or Snow Leopard Server as a Guest, but not Leopard or Snow Leopard desktop versions. You can order a free trial version of Snow Leopard Server here http://seminars.apple.com/contactme/SnowLeopardServerEvaluation/ (just got mine in the mail today.)
The trial version serial number expires January 30, 2010, but you may be able to trick it in the future by setting back the system clock (can't confirm), if you only needed to use it informally. My experience so far of running an SL Server guest on an SL desktop host is rather slow and not as polished as I expected.
Actually that's not the case. Microsoft does allow running its consumer operating systems in virtual machines. Its server OSs have a separate virtualization licensing scheme. MS seems to not allow installing the same Windows license twice, in both a virtual machine and a real bootable install. But that's different than Apple saying no consumer version of OS X can ever be installed in any virtual machine.
It is possible to have Mac OS X (desktop version) virtualized by making specific corrections to darwin.iso and virtual machine files. I did that myself, however I do not recommend to do that, because you will never receive proper Parallels Tools version and necessary updates, it will not work well!