Holding down the iPhone 4S home button works to call on Siri even if the phone is locked. No need to unlock and possibly type in your device password.
Update: If you use a passcode on your device and want to disable the ability to call on Siri from the Lock Screen, go to Settings > General > Passcode Lock, the option is there.
Voice Control in iPhone OS 3.0 took over the iPhone’s “press-and-hold the Home button to do something” ability, so the method to force-quit a stuck app has changed. Now you have to hold the power button at the top as if you’re powering down the phone. When you see the red slider, hold the Home button for a couple seconds. You’ll be taken out of the power down mode, and if all goes well, your app will quit.