I started Sunbird version 0.3a2 with fresh settings.
File -> Subscribe to Remote Calendar -> Select Remote ->
* WebDav
location:
http://server/service/home/accountname/calendar?fmt=ics
Give the Calendar a name: zimbra
Then you will be promted for your username and password.
Now you will see existing calendar entries from zimbra.
Create a new event (Make sure you select the calendar zimbra !)
Now if you view your calendar in the zimbra UI and refresh, you can see it's synchronized
