I don't know why its not known but you don't have to download any apps in order to take sceenshots on your phone.... Not sure if it works with all Droid phones but for my EVO 3D ... Simply press and hold the power button, then quickly tap the home screen button... You see a notification at the bottom of the screen say "screenshot saved to camera shots"......

Sent from my EVO 3D using DroidForums