Another platform, another GUI
While currently you would have to rewrite the UI for each platform to get the nicest result (as several people have pointed out), this is, in principle, unnecessary exertion: The user interface is (by definition) platform independent, and it is very easy to imagine a high level description of the user interface that isn't tied to any particular framework (not necessarily an API). Since such descriptions aren't mainstream (yet?), the problem currently becomes finding a framework that runs on all target platforms. At least you can then work to a single API. If a suitable one can't be found, or the compromises are unacceptable... yes, the effort must be duplicated... D'oh!