Remember Steve Jobs' first days back at Apple in 1997 as Interim-CEO-for-Life? Trying to save the company, Steve got Bill Gates to invest $150 million in Apple and promise to keep Mac Office going for a few more years in exchange for a five-year patent cross-licensing agreement? The idea in everyone's mind, of course, was that Microsoft would grab lots of Apple technology, which they probably did, and it quite specifically ended an Apple patent infringement suit against Microsoft. But I'm told that the exchange wasn't totally one-way, that Apple, in turn, got some legal right to the Windows API.
That agreement ran for five years, from August, 1997 to August 2002. Even though it has since expired, the rights it conferred at the time still lie with the respective companies. Whatever Microsoft grabbed from Apple they can still use, they just aren't able to grab anything developed since August 2002. Same for Apple using Microsoft technology like that in Office X. But Windows XP shipped October 25, 2001: 10 months before the agreement expired.
I'm told Apple has long had this running in the Cupertino lab -- Intel Macs running OS X while mixing Apple and XP applications. This is not a guess or a rumor, this something that has been demonstrated and observed by people who have since reported to me.
Think of the implications. A souped-up OS X kernel with native Windows API support and the prospect of mixing and matching Windows and Mac applications would be, for many users, the best of both worlds. There would be no copy of Windows XP to buy, no large overhead of emulation or compatibility middleware, no chance for Microsoft to accidentally screw things up, substantially better security, and no need to even take a chance on Windows Vista.