There may be a time when you need to “force quit” an application on the iPhone, this was achieved in iPhone 1x and iPhone 2x by holding down the Home button to “force quit” and were returned to the home screen.

With iPhone OS 3.0 that doesn’t work due to holding down the Home button activating Voice Control on the new iPhone 3G S states theiphoneblog.

So to “force quit” an application in iPhone 3.0 the user will need to hold the sleep/wake button until the slide to power off screen shows up, next press and hold the home button until the app quits, and job done.