If you’re using one of the new iPhone 3GS models you may have noticed it’s no longer possible to force quit an application by holding the home key down for 5 seconds. This worked on prior models but now holding the home key down will only bring up the voice control. Instead to force quit an application running on an iPhone 3GS press and hold the power button for 5 seconds. When the Quit/Cancel screen appears THEN you can hold the home key for 5 seconds and the application will force quit. When even that doesn’t work I’ve found the combination of holding home and power for 5 seconds will still force a reset.

Appmodo