After noting a several birthday events were missing from the Android Calendar on my Motorola Droid X, I started investigating.
I've noted a bug with Google Calendar events created using the 'Duplicate Event'. If an all day event is created in Google Calendar by via an exisiting event, using the 'Duplicate Event' dropdown. The new event appears correctly in the Google Calendar but doesn't seem to sync with the Android Calendar 2.2.1.
Having a repeat frequency set in the existing event from which the duplicate event is created also seems to cause some issues.
The same sync issue is not seen when a new all day event is created from scratch in the Google Calendar. Nor does the issue seem to occur when the duplicate event has a time range.