How do you port a Cocoa/Mac application to Windows? I mean how would you go about it? Assume the app was written with Objective-C and Cocoa, there's nothing fancy going on, no "engine" that could be factored out, etc.
Rewrite from scratch? I don't think there will be huge overlaps between the Mac and Windows codebases, right?
Are you aware of Cocotron? It looks like the project may have gone stale, but it's a good starting point anyway. It's a project to port Core APIs.
If your application is not cleanly separated (ie: a la MVC) then the only solution is a rewrite, I think.
The problem with Objective C is its very poor support on any platform that is not OS X. You can attempt to use the Cocotron, but I wouldn't consider it production ready yet.
For portability, a re-write is in order. With judicious use of standard C or C++ for the "core" of the application, you could still implement platform specific GUI code. If you don't like maintaining two GUIs, you can also try a toolkit such as Qt
Depending on which objects and framework you are using for your cocoa app, you might be able to get away with using gnustep, although the end result will probably look very weird to windows users, and the development environment might be a bit difficult to setup at first.
I have doubts about cocotron. Its not clear from the cocotron website that cocotron is actually production ready yet. Id suspect that it would be possible to start new app development and use cocotron constantly to maintain and test windows builds on the go. But to retrofit it into an existing project might be a much larger task. There are also no alternatives to cocotron - other than perhaps gnustep.
The practical approach to cross platform development involves developing the non gui components of your application, once, in C or C++. And then using a cross platform GUI library like QT - which is VERY good at generating and using native UI where possible or faking it where not. Please DO go to qt.nokia.com and download the latest build of QTCreator for windows and mac - See how the same QT application looks and feels very convincingly native on both platforms.
If QT doesn't provide a native enough solution, then you need to develop your GUI twice :- once in Cocoa, and once in Win32. The cocoa GUI would be in objective C of course, the Win32 GUI in C/C++.
Your non gui application code would - written in c++ - not be able to call Objective-C directly, but its not hard to write shim classes, implemented in .mm files - the provide a c++ interface, and wrap access to an objective c object or class.
You are also going to have to come up with an alternative to CoreData on windows - perhaps sqlite? Given that XCode has integrated support for the sqlite framework, and testing multiple code paths is, well, more work - perhaps dropping CoreData in favor of a common layer is a better approach?