Parallels and VMWare, the respective makers of Parallels Desktop and VMWare Fusion virtualisation solutions for the Mac have both admitted that they will not support Mac OS X virtualisation in their software.
The reason isn't technical though, as the two companies probably know how to make this work without the need of Apple.
The problem is that neither Parallels or VMWare want to strain their relationship with Apple by adding this feature without the Mac-maker's consent and, by the way, violating or encouraging to violate its current user license agreement as well as Apple's copyright regarding the protection system the company has developed so as to prevent Mac OS X from booting on anything that is not a Mac.
Virtualizing Mac OS X would allow PC users to run this system in a window or on full screen mode, which is probably one thing Apple doesn't like.
However, it would also be an interesting feature for the enterprise and professionals markets, as virtualisation is used as a flexibility enhancer for easily creating or simulating secure system environments. This technology has been clearly embraced by the Enterprise market during the past two years and is now widely used for production purposes.
It's also worth to note that, although many users of Parallels Desktop for Mac or VMWare Fusion will probably get a copy of Vista and run it on their Mac. The Windows Vista's user license agreement only allows Business editions of Vista to be used under an virtual environment. So Apple is not the only company that prevents its users from virtualizing its operating system.
Back in April 2006, MacosXrumors was the first to reveal that VMWare was porting its virtualisation solution to the Mac, a report that VMWare confirmed later in the year. In August 2006, it was also revealed by MacosXrumors that VMWare was seeking collaboration from Apple so as it can bring Mac OS X on a virtual environment.
At that time, our sources believed Apple could let VMWare virtualize Mac OS X, only if this feature was disabled on the Windows and Linux versions of their software.
Now it looks like the two companies - knowing they need close relationship with Apple so as to maintain improve and promote their products - are, at least for the moment, abandoning the idea.

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Isn't it already possible to
Isn't it already possible to virtualize Mac OS X on VMWare anyway?
You can do it but it's not
You can do it but it's not legal as you have to crack Apple's protection.
Anyway, even if VMWare brings support for OS X. I'm not sure it will be legal to use this feature without Apple's constent (implies changes in the user agreement).
Maybe Apple should allow OS X virtualization for solutions running on Mac or let Mac OS X Server be virtualized on either platform.
Mac OS X Server virtualization would definitely make sense... IMHO.
With all the support
With all the support Parallels is currently enjoying on Apple's web site, I can understand they don't want to mess things up.