code: function(map) {
          OpenLayers.Layer.Grid.prototype.setMap.apply(this, arguments);
          if (!this.tileOrigin) { 
              this.tileOrigin = new OpenLayers.LonLat(this.map.maxExtent.left,
                                                  this.map.maxExtent.bottom);
          }                                       
      }
docs: Method: setMap
  When the layer is added to a map, then we can fetch our origin
  (if we don't have one.)
  
  Parameters:
  map - {<OpenLayers.Map>}