Some aspects of the Cocoa development environment have now indeed been ported to Windows but only via Apple's Safari browser. Apple could have ported Safari to Windows using Microsoft's .Net tools (or Sun Java) but instead used their own Cocoa environment for Safari on both Mac and Windows. Of course the original beta release of Safari for Windows (Safari 3.0.0) had bugs in it -- the Cocoa libraries for Windows in Safari had to be created from scratch -- it was new Cocoa libraries and so they had to be developed from the ground up with lots of bugs that would not have surfaced if Apple used .Net for Safari but they wanted Safari for Windows to still be a Mac like browser. Of course that is not to say a .Net based Apple Safari browser in beta on Windows would be completely bug free but have less bugs. In any case Apple is fixing the bugs with new updates and remember its still a beta release (Safari 3.0.3 beta is latest version -- finalized version will a more stable release as opposed to existing experimental releases) -- so there is some Cocoa support in Safari for Windows but so far that's it.

This was a good article though on reasons why Apple should not make Cocoa for Windows.