Paul Woods, Edenspiekermann, on using Keynote as a layout and UI design tool:
In fact, when I first started working at ESPI last July, I was very surprised to find out that designers were using Keynote for laying out presentations. My surprise turned to alarm when I found out that they were also using it as a design tool to build UI designs for websites and apps. It turns out that I was absolutely wrong. Keynote is an incredibly powerful design tool. Less then one year later, I now rarely (if ever) use InDesign to layout presentations, and I have started using Keynote almost exclusively for any web layouts I do. And not just UX wireframes, but full UI designs.