Storyboards and Segues makes it really easy to design and prototype iOS apps at the same time
When I worked on the XBox 360, all the Interaction Design was documented in Visio diagrams which were manually translated into Flash prototypes for usability studies. I had wanted a solution like what XCode now offers with the recent integration of AppBuilder, and I even worked on an opensource Flash-based solution when I was with Artefact. This is basically the application design solution I’ve always wanted 🙂