Like everyone else, I ran across bfish's post on putting GCal in Thunderbird. I don't always have my calendar (pick iCal, iGoogle, or GCal) up, but I always have Thunderbird running. So I had to try it. It works as advertised!
Two comments. You can use the ICAL link for your Google Calendar, not just the XML link as noted in these directions; I used ICAL. The first calendar I created had the correct Google account filled in automagically, but the subsequent calendars did not (as noted in the comments).
And don't forget GCALDaemon if all you want is two-way synchronization between iCal (Sunbird, Lightning, Rainlendar, Evolution) and Google Calendar. It works for me, it gets new features all the time, and it is the best-documented version 1.0 F/OSS I've ever seen. And it worries my geek side a whole lot less than using Plaxo (with its spam history), also suggested.