It's possible, but Apple's left themselves enough breathing room that I doubt they'd be pushing their engineers so hard that they'd have to outsource the PowerMac's design to Intel.
We'll see.
OSx86 already runs on generic Intel hardware. There really isn't any technical reason that Apple couldn't use off the shelf parts for their systems.
Unless Apple has decided to change their paradigm, it doesn't sound likely to me. The motherboard is the core of Apple's hardware business, and Apple is, at heart, a hardware company. They write an O/S to sell hardware, not the other way around.
Also, keeping the motherboard proprietary lets them continue to control everything else.
Ipods, etc., will be spun off into another division, or into an entirely new company in a year or two, I think.