A simple Google Map Widget with a lat/lng marker. JS file included and required.

googlemap_location.js

/*
 * A simple Google maps Javascript widget which will display a map and
 * a marker with the ability to move the marker, then setting the 
 * lat/lng of the marker into the specified (or default) fields.
 */

var google_map_location = new function() {
    var jQuery;
    var init_options;
    var geocoder;
    
    var map, marker, latlng;
    

    /*
     * Options available:
     *  zoom: zoom level of map (default: 14)
     *  lat: initial lat of map
     *  lng: initial lng of map
     *  markerMovedCallback: callback function to use when marker is moved to populate a lat field.
     */
    this.init = function(options) {
        // Work around for when we have to use closures through django admin to pass in the jQuery object.
        jQuery = options.jQuery || $;
        init_options = options;

        latlng = new google.maps.LatLng(
                options.lat || 49.891235,
                options.lng || -97.15369
        );

        var myOptions = {
            zoom: options.zoom || 14,
            center: latlng,
            mapTypeId: google.maps.MapTypeId.ROADMAP
        };

        map = new google.maps.Map(document.getElementById('map_canvas'),
                            myOptions);

        marker = new google.maps.Marker({
                    position: latlng,
                    map: map,
                    draggable: true,
                    animation: google.maps.Animation.DROP
        });

        this.bindHandlers();

        // Attempt to get an initialLocation.
        if (options.geolocate && navigator.geolocation) {
            browserSupportFlag = true;
            navigator.geolocation.getCurrentPosition(this.hasPosition, this.noPosition);
        }
    }

    // Once we have received a poistion, center our map and marker to it.
    this.hasPosition = function(position) {
        latlng = new google.maps.LatLng(position.coords.latitude, 
                        position.coords.longitude);

        map.setCenter(latlng);
        marker.setPosition(latlng)

    }

    // If we have nothing, just show a friendly message
    this.noPosition = function(error) {
        jQuery("#map_message").html("Could not get current location. Please place marker over the Places location.");
    }

    /*
     * Accept an input field that contains a val() object from jQuery and tries to encode 
     * an address from it.
     */
    this.codeAddress = function(field) {
        var self = this;
        geocoder = new google.maps.Geocoder();

        geocoder.geocode({'address': field.val()}, function(results, status) {
            map.setCenter(results[0].geometry.location);
            marker.setPosition(results[0].geometry.location);
            self.populateLatLngFields(results[0].geometry.location);
        });
        
    }

    this.populateLatLngFields = function(point) {
        jQuery("#id_lat").val(point.lat());
        jQuery("#id_lon").val(point.lng());
    };
        
    this.bindHandlers = function() {
        var self = this;
        google.maps.event.addListener(marker, 'dragend', function() {
            var point = marker.getPosition();

            if(init_options.markerMovedCallback) {
                init_options.markerMovedCallback(point);
            } else {
                self.populateLatLngFields(point);
            }
        });
    };


};

googlemapwidget.py

from django.forms.widgets import Input
from django.utils.safestring import mark_safe

class GoogleMapWidget(Input):
    """
    Widget to display a Google Map on a page with the ability to move a marker
    around and fetch coordinates.
    """
    class Media:
        js = (
            'http://maps.google.com/maps/api/js?sensor=true',
            'static/js/google_map_location.js',
        )

    def render(self, name, value, attrs=None):
        """
        Attribute extras:
            width: width of the Google Map in pixels or percent. Must be the full string.
            height: height of the Google Map in pixels or percent. Must be the full string.
            markerMovedCallback: incase we don't want to populate the two default fields, let them specify their own callback.
            center: (lat, lng) coordinates in tuple format. If not provided, will auto-detect location if available.
            geolocate: Boolean. Default = True, If False, will not do any geolocation. Should be set to False if you provide a center() attribute
                    you do not want potentially over-ridden.
        """

        # Defaults, can be overriden by any same-key passed through `attrs`
        final_attrs = self.build_attrs(attrs)

        context = {
            'width': '600px',
            'height': '300px',
            'markerMovedCallback': 'null',
            'lat': 'null',
            'lng': 'null',
            'geolocate': 'true',
        }
        context.update(final_attrs)

        if 'center' in final_attrs:
            context['lat'], context['lng'] = final_attrs['center']

        html = u'''
        <input type='text' style='width: 400px;' id='map_search_text' /> <input type='button' id='map_search' value='Search' />
        <div id="map_canvas" style='width: %(width)s; height: %(height)s;'></div>
        <div id="map_message">Marker has been placed near the coordinates of your current location. Please adjust it to match the Places location.</div>
        <script type="text/javascript">
            //<![CDATA[
            var call_google_map = function(jQuery) {
                google_map_location.init({
                    'jQuery': jQuery,
                    markerMovedCallback: %(markerMovedCallback)s,
                    lat: %(lat)s,
                    lng: %(lng)s,
                    geolocate: %(geolocate)s
                });

                // Add some event handlers for our search box.
                jQuery("#map_search").click(function() {
                    google_map_location.codeAddress(jQuery("#map_search_text"));
                });

                jQuery("#map_search_text").keypress(function(e) {
                    if ((e.which && e.which == 13) || (e.keyCode && e.keyCode == 13)) {
                        google_map_location.codeAddress(jQuery(this));
                        return false;
                    }
                });
            }

            // Do a simple check to see if we're using this widget in the admin.
            if (typeof django != 'undefined') {
                (function($) {
                    call_google_map($);
                })(django.jQuery);
            } else {
                call_google_map($);
            }
            //]]>
        </script>
        ''' % context

        return mark_safe(html)