Recently I had to build a system that would take an address, pass it to google and return latitude and longitude values. The result is then stored for later use and display on Google maps.
However, firstly I found that the default XML geocoding service provided by Google that allows you to do bulk geocoding on a data source and the geocoding provided by their maps API/ AJAX library (GClientGeocoder) had huge differences in results. Most postal codes passed to googles XML geocoding service were miles out or just not recognised. Yet when using the same postcode in the maps API when calculated on the fly the results were more or less spot on.
More here:Google Maps API part 1: Accurate Geocoding for UK Postcodes
Lucky Larry's 