As notices MacRumors, a message on the french site MacGeneration forums posted by an anonymous, claims that Apple is developing its Cocoa framework and development environment xCode to Windows.
It is also reported in the forums post that Apple will port Safari to Windows the day the first Intel-based Mac ships.
The idea is to promote Mac development tools and framework to Windows developers, allowing them to create really universal binaries, independant of the processor and the operating system (Mac OS/Windows).
Earlier OS projects have been doing the contrary. On Linux, Wine simulates (and not emulates) a Windows environment and can run Windows applications. Apple had also a similar project at the early stages of development of Mac OS X (Rhapsody at that time) but it dropped the “Yellow Box” project with the release of Mac OS X.
There are however some reasons to doubt on the credibility of this forum post, time will tell. Meanwhile, I let you make your own opinion on it by giving it a read.