Short answer: Yes, it will. Long answer: Any x86 OS that boots on a generic BIOS-based PC will work, including Vista. (And no, Microsoft won't do anything to "stop" Vista from running natively on Apple hardware, e.g., via Boot Camp, or under virtualization, like Parallels Workstation.)
Actually, I tried it earlier and it doesn't work. The installer refuses to start because the virtual machine does not have power management. The error is something like "Vista can not be installed because it has detected your computer does not support ACPI".
Actually if you customize your install you can avoid the ACPI message and it will install and run. It will also depend on what aplha/beta Vista install you're talking about, if you're a legit beta tester you'll have the keys and install info you need, if you're talking about a pirated version then I, personally, wouldn't go near it. HTGH, peter