FYI, if you actually read the end user license agreement for Snow Leopard Client, it does not prohibit installing it in a virtual machine. Perhaps Apple intended it to say that, but if that was their intention their lawyers screwed up. Apple apparently has pressured VMware and Parallels into enforcing this prohibition even though it is not in the license agreement. However, that does not make it unethical for end users to do what is clearly consistent with the license agreement.
Nevertheless, for $20 it is easier to simply install Snow Leopard Server.