Additional very useful information:
http://code.google.com/apis/maps/documentation/javascript/services.html#ReverseGeocoding

Feel free to drop me a line if you have questions:
gzfl@bouncr.com

I am able to pass the latitude/longitude and reverse lookup like so:

getPostalCode: function (latitude, longitude) {
// tap into google maps api (Geocoder) to reverse lookup the postal code
var geocoder, latlng, streetValue, strippedVal, postalCodeValue, pcPattern = /[0-9]{5},/;
geocoder = new google.maps.Geocoder();
latlng = new google.maps.LatLng(latitude, longitude);
geocoder.geocode({'latLng': latlng}, function(results, status) {
if (status == google.maps.GeocoderStatus.OK) {
// test for results[0] which contains the postal code
// Example: 275-291 Bedford Ave, Brooklyn, NY 11211, USA
// results[3] could also be used
// Example: New York 11211, USA
if (results[0]) {
streetValue = results[0].formatted_address;
// match results against pcPatter regex *included , at end of zip to ensure correct data
strippedVal = new String(streetValue.match(pcPattern));
// strip the comma, pass the postal code to geoSetPostalCode
postalCodeValue = strippedVal.replace(',','');
log('SUCCESS---->\nLocationController.getPostalCode:\nPostal Code: ' + postalCodeValue);
LocationController.geoSetPostalCode(postalCodeValue);
}
} else {
// if status is anything other than OK, log and do fallback
log('ERROR---->\nLocationController.getPostalCode:\nStatus: ' + status);
LocationController.geoSetFallbackValue();
}
});
} //-- getPostalCode()