Apple doesn't want OSX running on any hardware that they cannot control. For current WINTEL systems there is always Darwin. But for specially made Apple WINTEL systems there is OSX WINTEL.

Apple wouldn't do it unless they had complete control over the hardware and could block access to it from other systems. So that only Apple Brand X86 systems will run it.

The reason for the X86 hardware is to build a cheaper Macintosh. With an AMD CPU and PC/ATX/FlexATX hardware Apple can build an OSX system for under $1000USD or even $600USD and compete in the Unix market against Linux, OpenBSD, Solaris, and even SCO Unix that run on cheaper hardware.

The PowerMac line would not be phased out, but I cannot see the X86 Apple systems running PowerMac code. I would imagine that Jobs needs cheap Unix workstations that also are easy to use and configure? At first they would run Darwin, until they can port the rest of OSX to it.