

Apple’s Overloaded iPhone Home Button - BSeward
http://www.lukew.com/ff/entry.asp?1382

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saurik
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.)

