To force quit an unresponsive app all you have to do is press and hold the Sleep/Wake button until you see the red slider. Normally, this is what you would do to power down your iPhone or iPod touch. In this case however, when the slider appears, release the sleep/wake button and then press and hold the Home button. If it works correctly, your iPhone will forcefully quit the unresponsive app and return you to your home screen.
I have found this especially useful on the rare occasion that Mail begins to choke on my multi-thousand message inbox. Sometimes this simple kick in the pants is all that is needed and is a lot quicker than a hard reboot followed by the waiting required during boot up.