IMHO, I always thought that Apple was first and foremost a Hardware company, and made sure it had great software to sell it with, if it couldn't compete in other ways. The reason Apple doesn't license OS X onto other hardware is to get people to buy Apple produced hardware.
If Apple were to license OS X to other PC hardware configurations, then firstly it would be put on a shift to Software-based as opposed to Hardware-based, and secondly, it would result in a mass of drivers needed for OS X, an ability to run equally well on ever so many different configurations (rather than, say, 10 configs) and people might start blaming the OS for hardware failure by their cheapest-possible PCs trying to run OS X.
Therefore, the Mac OS would be not only partially subsidised by the hardware, but also subliminally, the hardware's greatest marketing tool.
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