You guys really are oblivious. Why stop there? Why not port Win32 over to Mac OS X?
Microsoft WANTS you to stay on their platform. If they start porting all their libraries over to other systems the whole reason for staying on Windows falls apart.
Microsoft actually did port Win32 to Mac OS in the 90's. If you can, try to get a hold of the old Visual Studio (or whatever it was called; I think it was called Visual Dev) circa 1995, and look through the "Windows Layer for Macintosh" (WLM) documentation.
Or even look at the current Win32 header files, and you'll still see some #ifdef MAC portions that remain from those days. Microsoft ported Win32 to Mac OS, and provided cross platform remote debugging (debugging on NT, while the target app ran on the Mac).
But I think only Microsoft themselves actually used it, and it didn't get good results - it led to the infamous Mac Word 6 (was slow and looked more like a Windows app than a Mac app).