Darwin has been ported. Device driver support (as usual) lags though.
The NS apis (Cocoa) are extremely portable and in fact already run on top of windows to support WebObjects development on Windows. In fact, Apple builds all their stuff every week on a variety of hardware to make sure it stays portable.

But why would they cannibalize their hardware sales? If Apple were to decide to use Intel chips (ick) they would likely just use them in Apple hardware and you'd never know the difference.

I'm pretty sure they don't want to ship on Intel standard PCs for several reasons including:

1) The variation and general quality of implementation in PC land is a support nightmare requiring a vast number of engineers to work around.

2) Shipping on Intel while MS remains a proven monopoly gives MS ammunition during the penalty phase of the DOJ trial and we'd much rather have MS smacked down and de-fanged by the feds before entering the ring.

3) They'll need to figure out how to make up the missing margins on Mac hardware so Mac OS X for Intel is going to have to cost something like $600 per seat while it remains around $150 for Apple hardware.

4) Apps can be recompiled for either hardware - fat binary capability is already engineered into the app layout format. But getting app vendors to provide uniform cross platform support might be a problem.

5) Steve's fanatical about providing a great user experience and he might not feel like he can do that on the generally crummier and glitchier hardware pervasive in the PC world.