Apple's Shake-to-Shuffle feature made its first appearance in the fourth-generation iPod Nano, providing a means to shuffle song playback by giving the device a firm shake. The feature is now available for the iPod Touch and iPhone with the OS 3.0 firmware upgrade, giving people a handy method for skipping songs with a minimum of fuss.
Don't worry, music won't suddenly start playing every time you take a leap onto the sidewalk. You'll need to already be playing music for the Shake-To-Shuffle feature to work.
The feature can be switched off within the Music settings menu.