One of the greatest features in Flash Player 11.3 is the ability to accept full keyboard input while in fullscreen mode. This is really important for fullscreen games that need chat or other keyboard input. You can implement this feature very easily by using the FULL_SCREEN_INTERACTIVE mode. See below for the ActionScript code:

stage.displayState = StageDisplayState.FULL_SCREEN_INTERACTIVE;

Much like regular fullscreen mode, you need to add a parameter to your HTML code. See the code below:

<param name="allowFullScreenInteractive" value="true" />

When your application goes fullscreen, the user will be presented with an overlay at the top of the screen telling them that they are in fullscreen. They need to click the Allow button in order to be able to accept keyboard input. See the overlay below.

You can see a working demo of this new fullscreen mode here. Just be sure to have Flash Player 11.3 installed from Adobe Labs.