public function Main():void {
            NativeApplication.nativeApplication.addEventListener(InvokeEvent.INVOKE, onInvoke);
            ...
}
private  function onInvoke(event:InvokeEvent):void {
            toggleFullScreen();
            stage.scaleMode = StageScaleMode.NO_SCALE;
            stage.align = StageAlign.TOP_LEFT;
            ...
}
private  function toggleFullScreen():void {
            stage.displayState = ( stage.displayState == StageDisplayState.FULL_SCREEN_INTERACTIVE ) ?
                                                 StageDisplayState.NORMAL :
                                                 StageDisplayState.FULL_SCREEN_INTERACTIVE;
}

Taken from here: http://blog.ochodurando.com/2010/04/adobe-air-e-fullscreen/

Adobe AIR fullscreen on launch

ActionScript 3