code: function(callback) {
  
          this.closeDiv = OpenLayers.Util.createDiv(
              this.id + "_close", null, {w: 17, h: 17}
          );
          this.closeDiv.className = "olPopupCloseBox"; 
          
          // use the content div's css padding to determine if we should
          //  padd the close div
          var contentDivPadding = this.getContentDivPadding();
           
          this.closeDiv.style.right = contentDivPadding.right + "px";
          this.closeDiv.style.top = contentDivPadding.top + "px";
          this.groupDiv.appendChild(this.closeDiv);
  
          var closePopup = callback || function(e) {
              this.hide();
              OpenLayers.Event.stop(e);
          };
          OpenLayers.Event.observe(this.closeDiv, "touchend", 
                  OpenLayers.Function.bindAsEventListener(closePopup, this));
          OpenLayers.Event.observe(this.closeDiv, "click", 
                  OpenLayers.Function.bindAsEventListener(closePopup, this));
      }
docs: Method: addCloseBox
  
  Parameters:
  callback - {Function} The callback to be called when the close button
  is clicked.