This is also a very very old mac rumor that has popped up every few years. My personal bet is that Apple is feeling edgy over the new Xbox, which offers power PC chips (the CPU macs use) with more power and higher CPU speed than is currently offered in the highest level Powermac. Not a good move if they had wanted to maintain a strong relationship with apple.
A ported over Mac os is supposed to run Mac software at 80% of current mac operating speeds, though. Also, I doubt apple will make a move into the larger market, as microsof would crush them financially , and probably use soem pretty dirty tactics to do so. Apple has been happy with it's small niche market, and have been making record profits, along with dominating the portable music market because of their iPods.
Just my opinion, though, with alot of conjecture.