Update: Google no longer provides nor recommend this method. Please see official Google documentation.
Setup is fairly easy. Google provides a set-up tool to make the process quick and painless. Download the setup tool here. After downloading, unzip the file, and run the Calaboration application. Be sure to exit iCal before running the Calaboration tool.
After a few seconds the application will display your Google Calendars. Select the calendars you want to add to iCal and click the “Add to iCal” button. If your email address is not in your Leopard Address Book you will be prompted to do so.
Your Google Calendar will now appear in the list of calendars in iCal. Any changes you make to your Google Calendar in iCal will show up in your Google web Calendar.