If you are running iPhone OS 3.0 on your device already, you probably would have known by now that pressing the home button does not force quit applications anymore. 

Pressing on the home button for more than six seconds activates voice control on the iPhone 3G S. The need to change came there. 

Now if you want to force quit any applications that got stuck, press the sleep/wake button until the slide to power off screen appears and then press the home button until the application quits. 

Those of you who are yet to upgrade, be informed about this slight change.