I'm pretty happy with the approach of common x-plat core and then bindings to the native UI framework (Cocoa, WPF, etc.) on each platform. I'll bite the bullet of rewriting the UI layer for each platform if it means an absolutely great experience, you still save plenty from having that common core. Also, there's the nice side-effect that you end up learning the particulars of each platform, and by necessity end up with a really good architecture.

I do this with C#, not sure how many other languages support this approach. I wish it was all of them.