Worlds have collided. The universe has flipped to it's side and is now spinning counter-clockwise. Water flows up hill.
Windows XP will run on Apple's hardware.
And if that weren't enough to make you religious, Apple has provided the software and a simple step-by-step method of doing it.
I wouldn't waste time or money putting XP on my Mac (assuming I had an Intel Mac, which, if I sell my current Mac will surely come to pass). Vista? Perhaps six months to a year after it's released to ensure the bugs get worked out of it.
Personally, I'm not offended by this move by Apple. Yes, it's a bit like putting regular unleaded in a Ferrari, but overall I think it could make the Ferrari more accessable to the masses. And that could be a good thing...