I have done a lot of quick design projects in applications like PowerPoint, OmniGraffle, and lately Keynote. (I have Illustrator, but it can be tedious to use for small projects.) Apparently others do, too, and there are some great resources for prototyping with Keynote:
* [KeynoteKungFu](Keynote KungFu)
And for that iPhone application I am thinking about: [Glyphish](http://glyphish.com/).