If you are running an app (e.g. Pandora, Simplify Music) while your passcode-protected iPhone is locked, you can easily quit the app by tapping the Emergency Call button on the bottom left of the screen.
This saves you the trouble from having to enter in your passcode to quit an app that is running while your iPhone is locked.
Note that this hint only works with the iPhone; it does not work with the iPod touch.
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Mac OS X Hints
http://hints.macworld.com/article.php?story=20090506205217318