Mockapp.com has created both Keynote and PowerPoint templates of iPhone UI elements, and has made them available as free downloads. Say you had a dream in the middle of the night about the most awesome iPhone app that, to your surprise, no one has thought of yet. Instead of waking up in a deep sweat and scribbling said ideas on paper, you could dream them up on Keynote.
After mapping out your concept on Keynote, you could then pitch it to others in a Keynote presentation. The Keynote and PowerPoint templates include alerts, the iPhone keyboard, arrow icons, buttons, as well as a host of other UI elements.