Let's say it happens. Apple releases a version of Mac OS X for the PC-market. How long would it take until Mac OS X takes a significant percent of the market? MOST people, wouldn't feel a need to "upgrade" their OS, even if Mac OS X is, in most mac-users' opinion, a much better operating system/environment. Yes, tech-savvy people will probably 'try' it, as a proof of concept rather than choosing OS X as their main OS. Maybe in the future, with all the iPod-zealots and iPhone-maniacs out there, Mac OS X will become mainstream; but not today, or tomorrow, or in the next few years, I'd say.
Hard-Core Gamers (uhmm...naughty); Probably not going to happen. Think about it, PC-hardware is cheap (relatively speaking), you can get Windows XP/Vista pre-installed on almost anything, and even if you need to buy it, the cost will be offset by the "cheaper" hardware. Gamers are familiar with Windows/PC. Windows/PC allows for certain "choices" that cannot be had on OS X. When was the last time you heard of a Mac-enthusiast over-clocking his/her Core2Duo CPU? Running Mac OS X ? Also, games would still need to be ported, or wrapped, etc. Everyone hates ports. Only AAA-titles get decent ports nowadays (most of them on consoles). You think EA is going to start developing for OpenGL, when they have a HUGE Dx10 market out there? And, really, Apple is not going to adopt Dx10...
About Linux, as others have said, it doesn't just "work". I installed Ubuntu (7.10) on my PC, and...audio didn't work. Video Card was recognized as a standard VGA card. Wireless didn't work. Flash works whenever it wants to. And, don't be fooled, the community is terrible. If you need an ATI driver, and you happen to make the mistake of posting on the Ubuntu forums, you're going to get a flood of anti-ATI responses, and the general advise is to buy some other Video Card. I don't want to adapt to my OS, I want my OS to adapt to my needs. That's precisely what Linux needs, and what makes Apple so different. By "locking-in" what hardware is allowed to run OS X on, they make sure the OS WORKS as intended.