Even if your application is not designed to use iPhone-specific features like the camera, GPS, or Compass, when it runs on an iPhone it can encounter a number of unique situations. For instance, if a user accepts an incoming phone call while using your application, your application will quit. It must be prepared to handle this behavior. The right thing is to restore the user's state when they relaunch the application.
Additionally, if a user accepts an incoming phone call and then launches your application, the status bar will be taller than usual. Make sure your views respond correctly to this taller status bar. And if your application deals with audio, it must be prepared to handle the audio interruption caused by a phone call. Finally, because iPhone has more capabilities than iPod touch, it typically has less memory available for you to use, making it possible for applications to work properly on an iPod touch but run out of memory on an iPhone.