I've been creating a V3 Google map based on this example from Mike Williams http://www.geocodezip.com/v3_MW_example_map3.html

I've run into a bit of a problem though. If I have no parameters in my URL then I get the error "id is undefined idmarkers [id.toLowerCase()] = marker;" in Firebug and only one marker will show up. If I have a parameter (?id=105 for example) then all the sidebar links say 105 (or whatever the parameter in the URL was) instead of their respective label as listed in the XML file and a random infowindow will be opened instead of the window for the id in the URL. Here is my javascript:

        var map = null;
        var lastmarker = null;
                // ========== Read paramaters that have been passed in ==========

                // Before we go looking for the passed parameters, set some defaults
                // in case there are no parameters
        var id;
        var index = -1;

                // these set the initial center, zoom and maptype for the map 
                // if it is not specified in the query string
        var lat = 42.194741;
        var lng = -121.700301;
        var zoom = 18;
        var maptype = google.maps.MapTypeId.HYBRID;

        function MapTypeId2UrlValue(maptype) {
            var urlValue = 'm';
            switch (maptype) {
            case google.maps.MapTypeId.HYBRID: urlValue = 'h';
                break;
            case google.maps.MapTypeId.SATELLITE: urlValue = 'k';
                break;
            case google.maps.MapTypeId.TERRAIN: urlValue = 't';
                break;
            default:
            case google.maps.MapTypeId.ROADMAP: urlValue = 'm';
                break;
            }
            return urlValue;
        }
                // If there are any parameters at eh end of the URL, they will be in  location.search
                // looking something like  "?marker=3"

                // skip the first character, we are not interested in the "?"
        var query = location.search.substring(1);

                // split the rest at each "&" character to give a list of  "argname=value"  pairs
        var pairs = query.split("&");
        for (var i = 0; i < pairs.length; i++) {
            // break each pair at the first "=" to obtain the argname and value
            var pos = pairs[i].indexOf("=");
            var argname = pairs[i].substring(0, pos).toLowerCase();
            var value = pairs[i].substring(pos + 1).toLowerCase();

            // process each possible argname  -  use unescape() if theres any chance of spaces
            if (argname == "id") { id = unescape(value); }
            if (argname == "marker") { index = parseFloat(value); }
            if (argname == "lat") { lat = parseFloat(value); }
            if (argname == "lng") { lng = parseFloat(value); }
            if (argname == "zoom") { zoom = parseInt(value); }
            if (argname == "type") {
                // from the v3 documentation 8/24/2010
                // HYBRID This map type displays a transparent layer of major streets on satellite images. 
                // ROADMAP This map type displays a normal street map. 
                // SATELLITE This map type displays satellite images. 
                // TERRAIN This map type displays maps with physical features such as terrain and vegetation. 
                if (value == "m") { maptype = google.maps.MapTypeId.ROADMAP; }
                if (value == "k") { maptype = google.maps.MapTypeId.SATELLITE; }
                if (value == "h") { maptype = google.maps.MapTypeId.HYBRID; }
                if (value == "t") { maptype = google.maps.MapTypeId.TERRAIN; }

            }
        }

                // this variable will collect the html which will eventually be placed in the side_bar 
        var side_bar_html = "";

                // arrays to hold copies of the markers and html used by the side_bar 
                // because the function closure trick doesnt work there 
        var gmarkers = [];
        var idmarkers = [];

                // global "map" variable
        var map = null;
        // A function to create the marker and set up the event window function 
        function createMarker(point, icon, label, html) {
            var contentString = html;
            var marker = new google.maps.Marker({
                position: point,
                map: map,
                title: label,
                icon: icon,
                zIndex: Math.round(point.lat() * -100000) << 5
            });
            marker.id = id;
            marker.index = gmarkers.length;
            google.maps.event.addListener(marker, 'click', function () {
                lastmarker = new Object;
                lastmarker.id = marker.id;
                lastmarker.index = marker.index;
                infowindow.setContent(contentString);
                infowindow.open(map, marker);
            });
            // save the info we need to use later for the side_bar
            gmarkers.push(marker);
            idmarkers[id.toLowerCase()] = marker;
            // add a line to the side_bar html
            side_bar_html += '<a href="javascript:myclick(' + (gmarkers.length - 1) + ')">' + id + '<\/a><br>';
        }

                // This function picks up the click and opens the corresponding info window
        function myclick(i) {
            google.maps.event.trigger(gmarkers[i], "click");
        }

        function makeLink() {
            var mapinfo = "lat=" + map.getCenter().lat().toFixed(6)
                + "&lng=" + map.getCenter().lng().toFixed(6)
                    + "&zoom=" + map.getZoom()
                        + "&type=" + MapTypeId2UrlValue(map.getMapTypeId());
            if (lastmarker) {
                var a = "/about/map/default.aspx?id=" + lastmarker.id + "&" + mapinfo;
                var b = "/about/map/default.aspx?marker=" + lastmarker.index + "&" + mapinfo;
            } else {
                var a = "/about/map/default.aspx?" + mapinfo;
                var b = a;
            }

            document.getElementById("idlink").innerHTML = '<a href="' + a + '" id=url target=_new>- Link directly to this page by id</a> (id in xml file also entry &quot;name&quot; in sidebar menu)';
            document.getElementById("indexlink").innerHTML = '<a href="' + b + '" id=url target=_new>- Link directly to this page by index</a> (position in gmarkers array)';
        }


        function initialize() {
            // create the map
            var myOptions = {
                zoom: zoom,
                center: new google.maps.LatLng(lat, lng),
                mapTypeId: maptype,
                mapTypeControlOptions: { style: google.maps.MapTypeControlStyle.DROPDOWN_MENU },
                navigationControl: true,
                mapTypeId: google.maps.MapTypeId.HYBRID
            };
            map = new google.maps.Map(document.getElementById("map_canvas"),
                myOptions);
            var stylesarray = [
                {
                    featureType: "poi",
                    elementType: "labels",
                    stylers: [
                        { visibility: "off" }
                    ]
                },
                {
                    featureType: "landscape.man_made",
                    elementType: "labels",
                    stylers: [
                        { visibility: "off" }
                    ]
                }
            ];
            var options = map.setOptions({ styles: stylesarray });
            // Make the link the first time when the page opens
            makeLink();

            // Make the link again whenever the map changes
            google.maps.event.addListener(map, 'maptypeid_changed', makeLink);
            google.maps.event.addListener(map, 'center_changed', makeLink);
            google.maps.event.addListener(map, 'bounds_changed', makeLink);
            google.maps.event.addListener(map, 'zoom_changed', makeLink);

            google.maps.event.addListener(map, 'click', function () {
                lastmarker = null;
                makeLink();
                infowindow.close();
            });
            // Read the data from example.xml
            downloadUrl("example.xml", function (doc) {
                var xmlDoc = xmlParse(doc);
                var markers = xmlDoc.documentElement.getElementsByTagName("marker");
                for (var i = 0; i < markers.length; i++) {
                    // obtain the attribues of each marker
                    var lat = parseFloat(markers[i].getAttribute("lat"));
                    var lng = parseFloat(markers[i].getAttribute("lng"));
                    var point = new google.maps.LatLng(lat, lng);
                    var html = markers[i].getAttribute("html");
                    var label = markers[i].getAttribute("label");
                    var icon = markers[i].getAttribute("icon");
                    // create the marker
                    var marker = createMarker(point, icon, label, html);
                }
                // put the assembled side_bar_html contents into the side_bar div
                document.getElementById("side_bar").innerHTML = side_bar_html;
                // ========= If a parameter was passed, open the info window ==========
                if (id) {
                    if (idmarkers[id]) {
                        google.maps.event.trigger(idmarkers[id], "click");
                    } else {
                        alert("id " + id + " does not match any marker");
                    }
                }
                if (index > -1) {
                    if (index < gmarkers.length) {
                        google.maps.event.trigger(gmarkers[index], "click");
                    } else {
                        alert("marker " + index + " does not exist");
                    }
                }
            });
        }

        var infowindow = new google.maps.InfoWindow(
            {
                size: new google.maps.Size(150, 50)
            });

        google.maps.event.addDomListener(window, "load", initialize);

And here is an example of my XML formatting

<marker lat="42.196175"
        lng="-121.699224"
        html="This is the information about 104"
        iconimage="/about/map/images/104.png"
        label="104" />