I think this is the greatest thing ever! It is obviously undisputed that Windows machines are by far the most vulnerable machines in the world, but it is much more satisfying when a branch of Microsoft openly acknowledges this as well!
Check out the Top 10 Questions About Virtual PC from Microsoft, then scroll down to Question #10, "Does Virtual PC make my Mac more vulnerable to computer viruses?"
Indeed, in the answer, Microsoft openly acknowledges that Mac users are obviously exposed to the same vulnerabilities of a PC while they are running Virtual PC. Obviously--thankfully--these viruses will not be able to operate while any virtual machines running Windows are deactivated, and even if viruses do propagate to a Mac volume (by means of shared folders, for instance), they will not be able to run under the Mac environment. Nevertheless, the door is open for Mac volumes to be littered with virus files if a virtual machine running Windows is infected on Virtual PC. Indeed, this happened to me a few years ago, and it was a pain, because the virtual machine running Windows littered my Mac with virus files.
Microsoft is certainly forced to make such acknowledgments, and it is oh so sweet!