Boosman's response is far better than Cringely's column in pointing out the real problem: device driver management.
My experience with OSX drivers is that Apple barely gets enough support from device manufacturers (DMs) to stay above water. In some cases they bring development in-house to try to improve quality. Doing so in the Darwinistic land of PC hardware is impossible: the DMs must provide good drivers. Getting OSX marketshare up to the 25-50% level necessary for DMs to pay real attention will require years. During that time, OSX-on-nonApple-HW customers would provide a stream of complaints that would tarnish Apple's reputation but, more importantly, would slow down their development of OSX and give Microsoft a chance to catch up.
I personally would love to run OSX on other hardware right now, but PC hardware is getting _so_ commoditized that prices are falling to the point where the human cost of a poor operating system may outweigh the marginal cost Apple charges for their hardware for many people.
Apple is now 100% on that commodity train and as long as their marginal cost stays rational, they'll slowly grow marketshare.