Application Mockups
When specifying and communicating with people regarding different options/ways of implementing application UIs, I often take screenshots from existing applications and super-impose proposed data (using Photoshop) so everyone can get a feel for things prior to implementation.
Teehan+lax also use this approach and have produced an iPhone specific set of images that allow you to mockup your own UI. I think this approach is particularly pertinent for the iPhone because there are sometimes several ways to do the same thing - and some are much more iPhone-esque than others.
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