It's been addressed many times...
Why it can't be done:
There is a ROM chip in Apple hardware that OS X recognizes, and without it, OS X will not boot.
Why it won't be done:
Apple is not a software company. They make the majority of their profits from hardware, not from software. If Apple were to sell a PC version of OS X there would be zero incentive to buy Apple hardware, resulting in significantly less profits.
Plus, because Apple makes both the hardware and software, there is a much smoother user experience.
EDIT: I must be slow. 🙁