iPad is becoming an efficient platform for quick and dirty user interface design. The article on thenextweb.com discusses the 5 popular iPad wireframing apps.
I personally use OmniGraffle, as 1) The iPad interface is very intuitive and simple. 2) I can edit the same illustration both on iPad OmniGraffle and Mac OS X OmniGraffle. 3) I can export my illustrations to Microsoft Visio or most other vector editing apps out there.
Here is the original article: 5 iPad Apps for Wireframing & Brainstorming