Rumor has it that Parallels and VMware both have working code to run OS X VMs, but won't add the code to their released products to stay on Apple's good side.
I have not been able to feed my install disks to either and create a working OS X installation directly, but I have had VMware OS X VMs that came from the hackintosh world. I have used Tiger 10.4.10 and Leopard 10.5.5. The problem for Apple is that the VM files work just as well on VMware Workstation on a PC running either Windows or Linux, and Apple doesn't want that.