code: function() {    
          var bounds = null; 
          
          var length = this.grid.length;
          if (length) {
              var bottomLeftTileBounds = this.grid[length - 1][0].bounds,
                  width = this.grid[0].length * bottomLeftTileBounds.getWidth(),
                  height = this.grid.length * bottomLeftTileBounds.getHeight();
              
              bounds = new OpenLayers.Bounds(bottomLeftTileBounds.left, 
                                             bottomLeftTileBounds.bottom,
                                             bottomLeftTileBounds.left + width, 
                                             bottomLeftTileBounds.bottom + height);
          }   
          return bounds;
      }
docs: APIMethod: getTilesBounds
  Return the bounds of the tile grid.
  
  Returns:
  {<OpenLayers.Bounds>} A Bounds object representing the bounds of all the
  currently loaded tiles (including those partially or not at all seen
  onscreen).