It is only the Server version of Mac OS X 10.5 and Mac OS X 10.6 that can be legally virtualized! Also, VMware Fusion is designed to not run the Client version of Mac OS X 10.5 and Mac OS X 10.6!
Additionally, even though VMware Fusion is capable of running Mac OS X 10.7 and OS X 10.8 you can only legally virtualize it while running under the same Host OS on Apple-branded hardware. Meaning you may not virtualize Mac OS X 10.7 while running under OS X 10.8 and you may not virtualize OS X 10.8 while running under Mac OS X 10.7. Note that I said "may not" (as in conformance with the SLA) not "cannot" as in the ability not to do so! 