Originally discovered on iPod Touch Fans (and also on 9to5), it appears that the new 4th generation iPod Touch actually contains a motor that allows it to vibrate. The motor is shown in the FCC tear down photos of the device.
Apple notes the vibration alert in their iPod Touch Accessibility page:
If somebody wants to start a video call with you, you’ll receive an invitation -- along with a vibrating alert -- on your iPod touch asking you to join. Simply tap Accept, and the video call begins.
Vibration has been an iPhone-only feature since the original launch in 2007. The vibration has been used for silent alerts as well as a form of force-feedback in many iPhone games.
