Apple has inserted arbitrary hardware requirement restrictions in their OS products. In other words, while of course there's hardware that legitimately lacks the ability to run the thing, there's other hardware that can, but they have chosen not to allow it to happen. Mac OS X 10.5, for example, cannot officially be installed on anything other than a G5, or a G4 system of something like 600 or 700MHz, if I recall correctly, and also has a certain minimum RAM requirement. However, it's actually capable of running on ANY G4 system (as long as you have at least 256MB of RAM, though 512MB is better).
There have been previous instances of this with Mac OS X, and instances with Classic Mac OS as well.
This is all in addition to not being able to install a legally-purchased copy of Mac OS X on whatever supportable hardware you feel like. (Yes, I know you can with Hackintosh-type stuff, but you can't do it out of the box).