Why can't you run Mac OS on a Windows PC from within a VMware or Parallels virtual PC? Why does the Windows Vista EULA only allow the virtualization of Vista Ultimate and Business? It's all to do with both companies wanting to control virtualization in order to protect profits.
More controls, more restrictions, more limitations, more DRM Let's begin by looking at Mac OS X. Both VMware and Parallels have expressed a keen interest in bringing a virtual PC to the Windows platform that could run Mac OS X but neither company is willing to come up with the goods without the OK from Apple, and an OK from Apple is a LONG way off. Apple sees a commercial sense in being able to run Windows and Linux on Apple hardware but sees no reason to jeopardize hardware sales Apple is, after all, a company that makes most of their money from products, not hardware or software. Also, VMware and Parallels know, as does everyone else who keeps an eye on Apple, that Jobs and Co are very quick to turn to lawyers. Neither company want to attract that kind of attention, especially Parallels which currently enjoys being on Apple's "nice" rather than "naughty" list.
Now what about Microsoft's attitude to virtualizing Vista? Well, here the devil is in the EULA. To cut a long story short, Microsoft doesn't want the cheaper versions of Vista (Home Basic and Home Premium) installed into a virtual environment. If you want to do that Microsoft wants you to give them a little more cash and buy Ultimate or Business. Oh, and before you ask, there's no technical obstacle to installing Home Basic or Home Premium into a VMware of Virtual PC environment. The limitation exists only in the EULA. If you really want to experiment with Vista in a virtual environment you can either take advantage of the generous 120-day grace period before having to activate it and just demo it or cough up for a Technet Plus Direct subscription and get access to ISO downloads and product keys that allow you to have 10 installations.
Now, Microsoft's limitation that restricts you to running only Vista Ultimate or Business in a virtual PC is annoying, but at least you can do it, you just have to throw some cash at the problem (or you can ignore the EULA ... at your risk). But Apple's limitation on virtualization is a serious limitation platform weakness. Even being able to run Mac OS X guests within an OS X host would be a very useful feature indeed, but Apple doesn't seem to think that users want this. Being able to run Mac OS X on a Windows PC (or non-Mac hardware) would be really useful too but the reason why Apple doesn't want this to happen is pretty clear - it would kill their hardware sales. If you want to run Mac OS, Apple wants you to buy the hardware to go with it. With them you're not buying hardware or software, you're buying an Apple product.
But this isn't to say that you can't run Mac OS X from within VMware. In fact, if you fire up your favorite browser and visit a search engine and look for "VMware OSX image" you'll find that it's already possible. You have to download these files from dodgy sites, rely on an image that's been patched (maybe even patched in a bad way) and walk on the dark side of the law, but it's certainly not impossible - these images will run happily in the free player that VMware offers. What you're not going to see is a quick, simple and supported way to virtualize Mac OS. Rumors have it that Apple would have been willing to allow VMware to virtualize Mac OS X as long as this feature was not present on Windows and Linux versions of VMware products.
If you want to emulate a Mac, rather than virtualize the platform, then there are a number of open source projects that allow you to do this, as long as you're willing to go through varying degrees of hassle.
Virtualization has huge upsides for both home and business users, but both Apple and Microsoft want to have control over what users can or can't do with the platforms. More controls, more restrictions, more limitations, more DRM.