http://www.youtube.com/watch?v=V-xWMN5GrC0

and the corresponding code taken from Dustin Diaz's blog
Code:
YAHOO.util.Event.onDOMReady(function() {
  var h = YAHOO.util.Dom.getViewportHeight() - 1; // subtract a pixel so we don't get scrollbar
  var w = YAHOO.util.Dom.getViewportWidth()  + 590; // add width of M-velope
  function f (e) {
    var Anim = new YAHOO.util.Motion('envelope', {
        points: {
          by: [0, h]
        }
      },
      0.75,
      YAHOO.util.Easing.bounceOut
    );
    Anim.onComplete.subscribe(function() {
      window.setTimeout(again, 1000);
    });
    Anim.animate();
    function again (e) {
      var Anim2 = new YAHOO.util.Motion('envelope', {
          points: {
            by: [w, 0]
          }
        },
        0.5,
        YAHOO.util.Easing.backIn
      );
      Anim2.animate();
    }
  }
  window.setTimeout(f, 200);
});
anyone up for an Ext version?