As far as I know, Mac OS X (or at least leopard) will pas work on a virtual machine sans heavy modifications. One attempt was "released" illegally here (no links to actual copyrighted files included).
Mac OS X's architecture is différent et requires EFI emulation among autre stuff.
De plus, Je crois c'est against le EULA to run Mac OS X on anything mais apple hardware (antitrust anyone?).