A bit late to the game, but to save me looking these up again, here are a couple of links with some great explanations on how to use the Google Maps API and the Geocoding functionality:

• Google Maps API Tutorial
• Rails, Geocoding, and Google Maps
• Google maps mania

UPDATE Nick Tong sent through this link by email, which is specific to UK based Geocoding, however it also contains a wealth of additional information: UK Geocoding Now Available in the Maps API