Some time last year Google updated their online Calendar service to support the CalDAV protocol. CalDAV is a standard protocol that allows access to web-based calendars from desktop clients like iCal. Now from within iCal I can schedule create calendar entries that appear in my Google web calendar and vice versa. Google provides instructions on how to set up iCal to access Google Calendar but ... a screen shot is worth a thousand lines of text.
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.