private function init():void
{
addEventListener( Event.ENTER_FRAME , delay );
}

private function delay( e:Event=null ):void
{
removeEventListener( Event.ENTER_FRAME , delay );

var w:Number = stage.stageWidth;
var h:Number = stage.stageHeight;

}

/* Timer used to delay loading of Flash move by milliseconds to fix bugs related to IE */
var _ieDelayTimer:Timer = new Timer(50, 1);
_ieDelayTimer.addEventListener(TimerEvent.TIMER, onIETimer);
_ieDelayTimer.start(); // Timers in AS3 must be explicitly started

/* Delays the movie from loading by 50 milliseconds to allow everything to initialize and protect against known bugs in IE
*/
function onIETimer(e:TimerEvent):void {
_ieDelayTimer.removeEventListener(TimerEvent.TIMER, onIETimer);
init(); // put the start of all my code initialization, setting params, listeners, stagewidth vars etc in this function
}

http://www.stevensacks.net/2009/01/20/firefox-3-mac-flash-bug-stagewidth-and-stageheight-are-0/

I guess the best workaround is to have a resize handler on the stage and deal with it whenever the reported dimensions change (whether this is because the stage was resized or because the flash player finally figured out what size it was meant to be)…

Why not use a resize handler? When the stage size is initially 0 the Flash Player itself usually keeps on triggering the stage.resize event until it receives its current value.