Sean Heber over at the Iconfactory has just published a really awesome project called Chameleon; it's basically an open source re-implementation of UIKit for Mac OS X. With this, you can port apps from iOS to the Mac really easily using all the CoreAnimation-powered UIKit classes you know and love. You can even create a hybrid UIKit and AppKit application that integrates the best bits out of both the Mac and iOS development environment. Twitterrific 4 for Mac uses this framework to share its codebase across the Mac and iOS.
I was lucky enough to get an early chance to port an app to Mac OS X using this, and it blew me away. In fact, I was able to get several apps up and running with very little effort. All the localization work you've done works perfectly too.
SameGame for Mac uses Chameleon, and was ported in a couple hours - a perfect example of how easy to use Chameleon is. It should (approval pending) be on the App Store soon.