/***********************
Method 1: On Added handler
***********************/
addEventListener( Event.ADDED_TO_STAGE, function( e:Event ):void  
{  
     stage.addEventListener( MouseEvent.MOUSE_MOVE, myMouseMoveEvent );  
});  

/***********************
Method 2: The system manager parents all displayable elements within the Flex application.
***********************/
systemManager.stage.addEventListener( MouseEvent.MOUSE_MOVE, myMouseMoveEvent );

http://www.alwaysgetbetter.com/blog/2008/11/09/accessing-the-stage-in-flex/

There are times when you need to access the stage property before onCreationComplete.  Here are two ways to do it.

Flex: Accessing the stage

MXML