Adobe released a beta version of Flash Player 9 that adds new Full Screen capabilities to the Flash Player. This feature was primarily introduced into the update so that video players can go fullscreen.Full Screen mode is activated using either Actionscript 2 or Actionscript 3 by setting the value of a new property
Stage.displayState, it can only be set in response to a mouse or keyboard event, which stops malicious use of this new feature. Keyboard events do not occur when the Flash Player is in full screen mode, so input fields will not work.
More information on fullscreen mode can be found at Wiki - Full screen mode documentation.