Ok, here’s the lowdown:
You CANNOT and will not, ever, be able to run Mac OSX properly on a IBM PC. For starters they use different TYPES of processing units, let alone actual differences.
You can emulate the cpu, as people have pointed out; but it’s so ****ing slow it’s unstably unnusable.
Now, the core of Mac OSX, as people have noted, is based on *BSD; and this is the good bit, you CAN run the core on an x86, the core is codenamed Darwin, and the port is darwin-x86 or gnu-darwin, which is striving to port the macosx bsd core to x86. The bit that most people like about Mac OSX is the interface, either jaguar/panther or tiger, and the windowing system; these will probably never ever ever be released by apple as they are fairly snazzy. If you want perty interfaces look into using an alternative shell for xp or wait for Longhorn.
Hope that clears things up a bit.