The main product would be the PowerPC based Macs. The secondary platform would be MacX86 based. The PowerMac would not run MacX86 code and the MacX86 would not run PowerMac code, but with a switch in OSX you could compile for either platform. It would not be like the 68K Macs verses the PowerMacs, it would be a dual system support.
Apple's Benefits of the MacX86:
#1 Cheaper Hardware
#2 Less chance of a chip shortage like the PowerPC based Macs
#3 Able to dual-boot to Windows, Linux, etc.
#4 Able to run X86 NeXTStep software. At one time NeXTStep was written for Intel processors. Why abandon all that code?
#5 Apple can compete against Gateway. Dell, IBM, Compaq, and others for cheaper PC systems. The MacX86 can run the same software as the rest of them.
#6 Apple will most likely tweak the PC ROM to incude code converted from the PowerMac ROM. Maybe Quickdraw and other features will be used in the MacX86 ROM?
#7 The lower price will have Apple compete against cheapie Wal-Mart sold Linux boxes and other cheaper systems that run Linux, OpenBSD, etc.
#8 It will be sold to Unix shops that normally buy WINTEL hardware and run Unix on them as a workstation. OSX for MacX86 would give a low cost Unix Workstation that is easier to configure and install than your usual brand of Unix. All of the Unix features without a huge cost of the hardware.
#9 It would be a big boon for Pre-Installed Unix systems to have Apple sell them via the web store. So people can run Unix via OSX on X86 systems without spending a lot of money on PowerMac or more expensive systems.
#10 Apple could ultimately make a PCI expansion card to let generic PC clones run OSX, sort of like a dongle device. That way they can curb piracy and control what systems get to run OSX.
The PowerMac would not be dead, but instead this would be an alternative to a PowerMac and/or Unix/Linux/OpenBSD systems.