Not necessarily. If this happens, and that's still a major "if", I expect Apple will tie the OS to the hardware, much like they did back in the 68000 days. Both Amiga and Mac ran on 680x0 back in the day, but unless you laid your hands on (an illegal copy of) the Mac ROMs, you weren't running Apple's OS on your Amiga.
rdb3! I thought you were married... and not in Massachusetts!
The General is SO right. But Apple's need to tie the OS to the hardware is all the more reason that this purported switch makes no sense. I'd be more ready to believe the alternate rumor now floating - that Apple will move to a dual-core G5.