According to the official iPhone Human Interface Guidelines recently published by Apple, only one iPhone application may run at a time, and third-party iPhone applications will not be able to run in the background, posing a potential challenge to developers. “Only one iPhone application can run at a time, and third-party applications never run in the background,” the document states.
“This means that when users switch to another application, answer the phone, or check their email, the application they were using quits. It’s important to make sure that users do not experience any negative effects because of this reality.
In other words, users should not feel that leaving your iPhone application and returning to it later is any more difficult than switching among applications on a computer.” These guidelines may limit the usefulness of apps which benefit from a persistent network connection; it is unclear whether Apple will allow select partners, such as AOL and their AIM client, to bypass these rules. [via Daring Fireball]