Why bring OSX APIs to Windows? It's simple. More and more of the most useful applications today are collaborative and social in nature. Think about iChat. Here is one application on OSX that is a nice spin on the idea of instant messaging and will be vastly improved on Leopard. However, to benefit from many of the features in iChat, the other party needs to be running iChat. That's a deal breaker since the majority of users are running Windows. With the OSX APIs available on Windows (perhaps even a full Cocoa environment), porting iChat will be trivial for Apple. Porting any app that has a heavy social or collaborative aspect to it will be trivial (and essential) for Apple.
And then what? Well, Apple could expose that portability layer to third party developers. They could even make it possible for independent software developers to leverage an iTunes like platform for software distribution in much the same way that iTunes is a music distribution platform today. That would be huge for the small software developer that must overcome many barriers to get people to visit their websites and trust them with their credit cards.
