First you have to subscribe to the Google calendar from iCal.
You look under Calendar Details on
calendar.google.com; you want the format. Google tells you, Please use the following address to access your calendar from other applications.
Right-click to Copy Link Location. Then, in iCal, under the Calendar menu, select Subscribe..., and paste in the URL.
This works - but note that iCal stores the link as
Now you can set up iSync to synchronise this calendar with your phone. Launch iSync, add the phone as a device if you haven't already done so. Select the phone icon and select which calendars to synchronise. Note that this is really copy, from Google Calendar to your phone, rather than sync, since events modified on your phone (or in iCal) don't change on Google Calendar. Maybe the iPhone will do better?