[udaaff]

Если правильно понял:

var x0:Number = stage.stageWidth - mc.width;
 
stage.addEventListener(MouseEvent.MOUSE_MOVE, stage_mouseMoveHandler);
 
function stage_mouseMoveHandler(event:MouseEvent):void
{
	mc.x = x0 - event.stageX * x0 / stage.stageWidth;
}