code: function(center, resolution) {
  
          var extent = null;
          
          if (center == null) {
              center = this.getCachedCenter();
          }                
          if (resolution == null) {
              resolution = this.getResolution();
          }
      
          if ((center != null) && (resolution != null)) {
              var halfWDeg = (this.size.w * resolution) / 2;
              var halfHDeg = (this.size.h * resolution) / 2;
          
              extent = new OpenLayers.Bounds(center.lon - halfWDeg,
                                             center.lat - halfHDeg,
                                             center.lon + halfWDeg,
                                             center.lat + halfHDeg);
          }
  
          return extent;
      }
docs: Method: calculateBounds
  
  Parameters:
  center - {<OpenLayers.LonLat>} Default is this.getCenter()
  resolution - {float} Default is this.getResolution()
  
  Returns:
  {<OpenLayers.Bounds>} A bounds based on resolution, center, and
  current mapsize.