The Unofficial Apple Weblog momentarily speculates that Safari for Windows could be based on Yellow Box for Windows, which was a port of OpenStep (the NEXTSTEP framework that became Cocoa) to Windows back in the end of the last millenium, before Mac OS X was ever released. But this is quickly quashed, as analysis of the code shows that Safari for Windows does not use Cocoa or Objective C.
I think that Apple has probably completely abandoned Yellow Box for Windows. This is, after all, a code base that is nearly a decade old, intended to run on Windows NT and Windows 95. It would take a lot of work to port the modern Cocoa framework, and even more work to make it suitable for Windows Vista.
I think that the new paradigm will be the integration of hypervisors into the OS. Parallels Desktop for Mac is an example of a hypervisor, which is already capable of running Mac OS X apps simultaneously with Windows apps, the way that the Classic environment allows you to run OS 9 apps at the same time as OS X apps, and sort of the way that you can run Aqua and X at the same time. And there is an open source hypervisor called Xen that vendors like RedHat want as part of the Linux kernel.
While porting Cocoa to Windows would certainly expose the framework to way more developers, it would also remove any incentive to migrate from Windows to Mac OS X, and Cocoa would probably be wrongly blamed for the security problems that are already inherent in the Windows platform.