Under iPhone 1.x and 2.x you could force quit an application by holding down the home button for a few seconds. In most cases, this helped quit an unresponsive application and it freeed up some memory as well.
Under iPhone 3.0 though, holding down the home button for a few seconds will activate Voice Control. So how can you force quit an application in iPhone 3.0? iPhone Download Blog reader VistaUser writes in a comment:
1) Hold down the Power and Home button until the “slide to turn off” appears.
2) Let go of both Power and Home button once the “slide to turn off” appears.
3) Once again, press and hold ONLY the Home button for about 5~6 seconds and this will force quit the app.
As VistaUser writes in his comment, this adds an extra step to everyone, especially those who are not on an iPhone 3G S. But sooner rather than later, some smart guy will come up with a way to add Voice Control to jailbroken 1st Gen and 3G iPhones so this will come in handy for everyone.