Google has opened up their calendar system so that you can have real two-way sync of your online Gmail calendars with iCal. You can set up multiple calendars with what appears to be full iCal functionality, and have them "push" changes to Google. You can read their detailed instructions for help getting things going. (You'll then need to click the Enable Google Calendar in Apple's iCal link on that page.)
With these new abilities, the workarounds mentioned in this hint and comments are no longer necessary.

