Only Apple iCal 3.x (on Mac OS X v10.5+) supports CalDAV sync.
To set up CalDAV support for Google Calendar in Apple's iCal, follow these steps:
You won't be able to invite or email guests to Google Calendar events within iCal if your address is not in your Address Book.
Your Google Calendar will now appear in iCal's list of calendars, and changes you make to your Google Calendar in iCal will be reflected when you sign in to Google Calendar.
Event information will automatically refresh every five minutes. You can change this interval by opening the iCal > Preferences > Accounts menu and selecting a new refresh interval under the 'Refresh calendars' drop-down. Keep in mind you can also force a refresh by selecting a calendar and pressing the Apple and R keys at the same time.
Calendars that are checked but disabled have already been added to iCal - you can remove them from within iCal.