We have been developing web sites since the early 1990's and are thoroughly experienced in terms of what will work, what's usable and the process to go from idea through to delivery. Easy. Part of the interest in developing an iPad app was to explore the design and development differences from the web.
Apart from some core functional requirements (what we wanted it to do) we started by looking at what other apps do and how they work: Mail, Twitter, Evernote etc then developed a rough prototype from live data to see how it felt. It worked OK so I took it for a test drive to Bhutan and Burma where it crashed occasionally but was mildly useful since there was almost no Internet connectivity there. This prototype gave us important clues as to how it felt to use it, and the experience of using it; all that finger swiping and screen rotation to contend with. It also gave us some insights into what was missing - some key stuff like global and user functions that needed priority.
We quickly decided that we couldn't simply evolve the prototype into a final app and actually expect it to deliver on our vision for the product, so we rapidly developed some paper wireframes which consolidated our thinking, allowed basic user testing and developed consensus about the direction we'd go in. These can be seen in this rather funny video which makes us laugh every time we see it.
But good design isn't just about functionality; it should 'feel' like a travel app and immediately be recognizable as one, so after some discussion the creative team developed an interface with some visual queues, one which in my view went too far. So we pared it back to something more minimalist and it became apparent that that wouldn't work either. Perhaps it is all subjective anyway, but where we ended up is a pleasant balance of usably functional with a clean visual language.
The creative process ever remains fascinating., but we found developing an iPad app very different from the norms of web site development. On the web, user interactions with a mouse are pretty well understood and the user interface elements available have standard rules around use which keeps you from making any obvious errors. But the iPad all this finger swiping is all new.
We almost certainly haven't got it right with version 1.0 but the process of learning how to get there has been important.