First of all Mac OS X 10.6 Client version is not allowed to be virtualized as it's not allowed by its SLA, although the Server version is, however legally virtualizable versions of OS X in a Virtual Machine may only be done when done on Apple-branded hardware and when done while running under the same version of OS X. Meaning as an example if you have Mac OS X 10.7 installed as the Host OS you can virtualize 10.7 not 10.8 as the Guest OS.
You could however natively dual boot the Mac between Mac OS X 10.6 and Mac OS X 10.8 as an example.