This is a great site to make a mockup of an iPhone app. Irregardless if you design the app in the end or just use it to prototype your ideas is up to you. Handy too though to help with the design phase.
From their site:
Any creation of a mockup through this website happens at your own risk. Note that your mockups are not protected in any way other than by a non-obvious URL. I can not be held responsible if you create a mockup and your competition figures out how to access it. Mockups can contain user-uploaded images and user-entered text. View at your own discretion. I do not watch, see or check this user-generated content in any way, and can not be held responsible for images uploaded by users of this site or text entered by users of this site.