UI Sketcher: The User Interface Sketching Tool iPad App
From my good friends at Box UK, UI Sketcher was:
Created by User Experience professionals and built upon the principles of Design Studio Method and Adaptive Path's Good Design Faster, UI Sketcher enables you to rapidly sketch, refine and share user interface ideas.
I saw an early demo of this app a good few months ago now. Whilst I'm not sure it's for me (i just don't get on with anything other than a pen and paper for my sketching), it's well worth a look. Buy it.