I think I’ve been waiting for this from the first day I know Google Calendar. There’re some third party solution for doing this, but I feel they’re too expensive. Gradationally, I gave up Google Calendar with my MacBook Pro iCal, because I always have my hands on my MBP.
The trick is you can export your existing calendar from iCal and then import it into your Google Calendar subscription in iCal! So from now on, you have everything in both your iCal and Google Calendar! 😉