Arguably, if Apple had ported their OS to x86 in the early 1990s, Windows would be dead in the water today. I've heard this hypothesis a few times.
However, since they didn't, they have to cling onto their proprietary platform. For this reason, I don't think porting OS X to x86 is now worthwhile.
They would definitely sell a few more copies of the OS, but it would just as well hurt their high-margin hardware business. For Apple, software is just a means to sell their machines.
Intel has about 75% of the CPU market share, and the PC systems business is cut-throat, but at least those competitors are not illegal monopolies. As a business, going head up against the Winblows monopoly is not something many people even seriously consider.
Even most of the Linux vendors are much more interested in making money on the server, and on services. The economies of scale for profit in Linux desktops just isn't there. As an example, I recall reading last year that SuSE loses money on each retail boxed OS they sell.