A friend once showed me this brilliant app for the iPhone that can be used to do interface design and prototyping for Objective C applications on the iPhone.

Personally, I have only just started using it to test ideas that I have when I have them, which is quite handy as more often then not you tend to come up with ideas when you are away from your computer.

Here is a very basic mockup I made just before in about 2 minutes:

Interface is only $9.99 on the App Store, and now comes with a feature to export your mockup interface to Xcode.