Tip: Force Quit Apps in iPhone OS 3.0
Due to some features of the iPhone 3G S (notably Voice Control, which requires holding down the home button), the shortcut to force quit an app in iPhone OS 3.0 is different from OS 2.x’s hold home forever method.
In iPhone OS 3.0, to force quit an application, hold the sleep/wake button (the button top of the device) until the “Slide to power off” menu comes up, then release the sleep/wake button and hold the home button. After a few seconds the power off menu will disappear and your app will be forced to quit.