Well, we can, but don't expect to be able to create a virtual OS X anytime soon. While Parallels and other companies offer software allowing Windows users to virtualize Linux and various other operating systems, Apple has made clear its desire for OS X to be run exclusively on its own hardware.
Although Apple's permission is not actually needed, both Parallels and main competitor VMWare have stated that they will not virtualize OS X unless given Apple's blessing. Parallels is committed to a healthy working relationship with Apple, and with good reason: its product is currently displayed in Apple stores and even took home the MacWorld 2007 "Best in Show" award. Although the idea of running two platforms simultaneously may seem daunting, the necessary security measures have already been implemented. Virtual machines work as isolated files, and as a result, viruses or spyware from a virtual machine cannot affect the main operating system.
Additionally, while Boot Camp users need to partition their hard drives, allotting a certain amount of disc space for each operating system, virtual machines do not require such overhead.
Parallels includes support for the newest versions of Windows Vista as well as all other releases dating back to Windows 3.1 (for those of you in the market for a blast from the past).
