code: function(opacity) {
          if (opacity != this.opacity) {
              this.opacity = opacity;
              var childNodes = this.div.childNodes;
              for(var i = 0, len = childNodes.length; i < len; ++i) {
                  var element = childNodes[i].firstChild || childNodes[i];
                  var lastChild = childNodes[i].lastChild;
                  //TODO de-uglify this
                  if (lastChild && lastChild.nodeName.toLowerCase() === "iframe") {
                      element = lastChild.parentNode;
                  }
                  OpenLayers.Util.modifyDOMElement(element, null, null, null, 
                                                   null, null, null, opacity);
              }
              if (this.map != null) {
                  this.map.events.triggerEvent("changelayer", {
                      layer: this,
                      property: "opacity"
                  });
              }
          }
      }
docs: APIMethod: setOpacity
  Sets the opacity for the entire layer (all images)
  
  Parameters:
  opacity - {Float}