A sampling of code for loading/unloading thousands of points as the user pans around the map.

googlemaps_loading_lots_of_points.js

function requestMarkers() {
  // no loading markers when zoomed way out
  if (map.getZoom() < 11) { return };
  // start dropping markers before the browser bogs down
  if (markers.length > 50) {
    dropSuperfluousMarkers();
  }
  $.ajax({
    // [snip typical ajax data request]
    success: populateMarkers,
  });
}

// add the markers from pointsData if they haven't already been placed
function populateMarkers(pointsData, status, xhr) {
  for (var i = 0, ii = pointsData.length; i < ii; i++)  {
    var lat = pointData.geom.x;
    var lng = pointData.geom.y;
    var marker = new google.maps.Marker({
      position: new google.maps.LatLng(lat, lng),
      draggable: false,
      animation: google.maps.Animation.DROP, // whee!
      flat: true, // disables some styling DOM elements for a faster marker
    });
    
    // hash the marker position
    coordHash = calculateCoordinateHash(marker);
    
    /*
    if we haven't seen this hash, add the marker and mark as seen
    
    without this, the markers array quickly grows unwieldy as duplicate points
    are loaded
    */
    if(seenCoordinates[coordHash] == null) {
      seenCoordinates[coordHash] = 1;
      markers.push(marker);
      marker.setMap(map);
    }
  }
}

// turn marker coordinates into a hash key
function calculateCoordinateHash(marker) {
  var coordinatesHash = [ marker.getPosition().lat(),
                          marker.getPosition().lng() ].join('');
  return coordinatesHash.replace(".","").replace(",", "").replace("-","");
}

// remove markers that aren't currently visible
function dropSuperfluousMarkers() {
  mapBounds = map.getBounds();
  for (var i = 0, ii = markers.length; i < ii; i++)  {
    if (!markers[i]) {continue};
    if (!mapBounds.contains(markers[i].getPosition())) {
      // remove from the map
      markers[i].setMap(null);
      
      // remove from the record of seen markers
      coordHash = calculateCoordinateHash(markers[i]);
      if(seenCoordinates[coordHash]) {
        seenCoordinates[coordHash] = null;
      }
      
      // remove from the markers array
      markers.splice(i, 1);
    }
  }
}

// clear all markers from the map, empty the markers array and seen markers
function clearMarkers() {
  for (var i = 0, ii = markers.length; i < ii; i++)  {
    markers[i].setMap(null);
  }
  markers = [];
  seenCoordinates = {};
}