Apple’s Overloaded iPhone Home Button(lukew.com)
lukew.com
Apple’s Overloaded iPhone Home Button
http://www.lukew.com/ff/entry.asp?1382
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13) When in an application, and while the "power down controller" is up (you can bring it up by holding the power button for a few seconds and then letting go), holding the home button for noticeably more than a few seconds will "force quit" that application. (Note: this functionality is accessible without first bringing up the power down controller on the original iPhone and the iPhone 3G, as they do not have a Voice Controls to bring up instead.)