Because of the way it is used, iPhone needs to be snappy and expects the same of your application. When your program is launched, you have to get your application open, preferences and data loaded, and the main view shown on the screen as fast as possible—in not more than a few seconds. At any time when your program is running, it may have the rug pulled out from under it. If the user presses the home button, iPhone goes home, and you have to quickly save everything and quit. If you take longer than five seconds to save and give up control, your application process will be killed, regardless of whether you are finished saving.
Beginning iPhone 3 Development
Dave Mark / Jeff LaMarche