Currently if you want to access a calendar with both Google Calendar and iCal, you can only share the calendar one way. So if you want to make modifications in both places, you need to create two calendars. One in iCal, and subscribe to it in Google Calendar. Create other in Google Calendar and subscribe to it in iCal.
It looks like Spanning Sync will make this integration even better. With Spanning Sync you can modify the calendars in either place, and have the changes reflected in the other.
It is not available yet, but will be entering a private beta next week, with a public beta after that.