Spanning Sync looks to be the holy grail of true, two-way calendar synchronization between Apple's iCal and Google Calendar. Yes: by 'two-way' I mean 'you can edit the same calendar in either place and the changes will synchronize.' Nasty translation I know, so maybe you should go watch their demo video to really get your mouth watering. In it, one of Spanning Sync's creators shows off how easy it is to link iCal calendars with equivalents in Google Calendar (bonus points for the ability to chose exactly which calendars shake hands), as well as moving an event in one calendar and having the change sync to the other. If this doesn't have you excited, please excuse me while I go check my blood pressure.
Spanning Sync's blog has more details, and its release is billed as 'coming soon.' Stay tuned to TUAW as well, as I was fortunate enough to get my name in on the private beta before the signup list closed, and I'll have a screencast and a review for you as soon as possible.