Here's an editorial about Apple's announcement of switching to x86 processors and a look at the future of Mac OS X. Apple’s recent collaboration with Intel to use its microprocessors surged a great deal of interest throughout the industry. Though there are several arguments spanning the market currently, one thing is certain – Apple has stated it will not let users install Mac OS X on non-proprietary hardware, even though the CPU will fully comply with x86 architecture. The only apparent way for Apple to achieve that is by using some form of protection technology, however, that may ultimately turn out to be a fatal strategy. More info at CTZ
I agree on some points of the article but I believe there are two major reasons why Apple won't allow Mac OS X to be installed on non-Mac hardware. Firstly because most of Apple's revenue comes from Mac hardware and secondly because it would ask a lot more effort to support all kinds of exotic hardware that is available for the x86 platform.
Apple is, compared to Microsoft, in a quite easy position for their OS development right now. While Microsoft provides lots of backward compatibility this is a bit less extensive on Apple's side. Currently Apple also has the advantage that their software only runs on their systems. This mean that they can do a lot more testing and if they would allow Mac OS X to be installed on other non-Mac x86 systems with AMD or Intel processors they would need to do a lot more effort to guarantee compatibility. It could hurt Mac OS X's stability and will likely also affect development time.