Well, the most underused feature, and it counts for every type of devices (iPhone, blackberry) is ... the Web browser and particularly Mobile Web apps running on it ;-)
Native apps has been dominating the Mobile landscape for a while, but nowadays, with HTML5 and some nice JS and CSS , Mobile Web apps are really ready to take over the market and compete with native apps. No learning curve (for the web developers), update whenever you want, bypass any app acceptance process, provide the content you want ...
This is the core concept of this book but in in the same time it shows also how you package it into a native app if you really want to!
The issues I have had with Web Apps in this fashion is that they don't give the same smooth UI experience that a native application does, even with HTML5. Anytime you jump from one "page" to another in the web app it has a jump to it and it shows.
In regards to iPhone apps, I think this is where Apple shines, is that they take a lot of time to make the native UI experience to be very rich that you can't match with other iPhone type apps like a web app.
However, if you want to write a single app that runs on all devices quickly, a Web App is the best approach.