Retrieve latitude and longitude of a draggable pin via Google Maps API V3

Either of these work

google.maps.event.addListener(marker, 'click', function (event) {
    document.getElementById("latbox").value = event.latLng.lat();
    document.getElementById("lngbox").value = event.latLng.lng();
});

google.maps.event.addListener(marker, 'click', function (event) {
    document.getElementById("latbox").value = this.getPosition().lat();
    document.getElementById("lngbox").value = this.getPosition().lng();
});

You might also consider using the dragend event also

google.maps.event.addListener(marker, 'dragend', function (event) {
    document.getElementById("latbox").value = this.getPosition().lat();
    document.getElementById("lngbox").value = this.getPosition().lng();
});