написал по быстрому, даже не проверял, теоретически должно работать

Код:
public class Baner extends MovieClip
{
private var dx:Number;
private var status:String;
private var m1:MovieClip;
public function Baner()
{
mc1 = getChildByName("clip_bg");
addListeners();
}
private function addListeners():void
{
stage.addEventListener(MouseEvent.MOUSE_OVER, mouseEvent);
stage.addEventListener(MouseEvent.MOUSE_OUT, mouseEvent);
}
private function mouseEvent(e:MouseEvent):void
{
if (e.type == 'MOUSE_OVER')
status = 'show';
else
status = 'hide';
dx = 0;
addEventListener(Event.ENTER_FRAME, eventShow);
}
private function eventShow(e:Event):void
{
dx += 0.01;
if (status == 'hide')
{
mc1.alpha -= dx;
if (mc1.alpha < 1)
{
mc1.alpha = 0;
removeEventListener(Event.ENTER_FRAME, eventShow);
}
}
else
{
mc1.alpha += dx;
if (mc1.alpha >1)
{
mc1.alpha = 1;
removeEventListener(Event.ENTER_FRAME, eventShow);
}
}
}
}