There are some (very expensive) tools out there that try to output code from your design, but they are incredibly complex to use. I export Mockups as XML, which, with some fancy XSL, could be turned in to MXML or XAML...which would at least define the structure of the application. Still, that's not the most time-consuming part of a development effort, but it would sure make for a good demo! :)

The nice thing about Interface Builder style tools is it doesn't generate actual code, it saves "freeze dried" versions of the objects. It's also free and very easy to use.