Check out this experience report on Cocotron:

The promise of Cocotron is a big one. Wrote a Cocoa app? Just add a new Xcode target, hit compile and out shoots a Windows version. Your mountains of Objective-C code now cross compiles and is no longer stranded on the Mac. You put your feet up, having just shipped a Windows app without ever touching a PC. Your cat brings you a freshly brewed cup of coffee. Read on for our journey from app to exe.