From what I've read in other threads, Apple did, at some stage, write a version of Mac OS X that would run on the x86 architecture (presumably when they hadn't decided what architecture they'd use in the future, before the G5), but it was never released. I see this as a good thing though, for two reasons.
1. If you could run Mac OS X on a Wintel machine, it would be less stable because Apple would have less/no control over hardware and it is harder to program an OS when you don't know what hardware it's using.
2. People would have no incentive to buy Macs since they have the software anyway. I understand that Apple's profits are mostly from hardware sales, so Apple would likely to bankrupt fairly quickly.