Mac Geekery has a lengthy article on why Apple shouldn't offer Mac OS X for normal x86 PCs:
Over 80% of Apple's sales relate to hardware (Form 10-K, Dec 2005 ) which leaves about 20% for other things, such as software and the music store. This software would be items such as Mac OS X, but also software (with higher margins) such as iLife, iWork, Final Cut Studio and Mac OS X Server. From today's numbers, software is not the most significant part of Apple's sales, even though some of the prices dance dangerously close to the price of new hardware. This begs the question "what if software was a significant seller?" Well, let's look at two companies that were created by ex-Apple employees who had the same idea..