I had noticed that most of my Home calendar, including birthdays and anniversaries, was empty on both machines I .Mac sync. No idea why, but the data was gone.
So I decided from now on I’d put birthdays into address book and use a birthday tool to create a calendar from it. I could not find my Calendar file so I used spotlight to search for “home” and wow, it found all of my home events! Double clicking each one opened it in iCal to show it had expired 2 years ago, etc, so I reset the alarms and the ending date (to never) and synced up with .Mac.
Whew! Still need to find where those calendars are kept, as I like the idea of generating the calendar from Address Book vs. trusting iCal, but Spotlight came through for once!