I am currently evaluating several services from Mac OS X SL server which do allow to use external calendars instead of using the internal Mac OS X CalDAV server.
Right now, I figured out that it is possible to configure such in Mac OS X server > Server Admin > Web > Settings > Wiki > External Web Services > Configure > Web Calendars > Calendar Server: zimbra.domain.com/prinicpals/users (SSL: yes).
After logging into my account in Mac OS X and clicking "calendar", everything seems to be fine as the website will ask me for the username and password for the area "Zimbra" using a basic authentication. I would even confirm the authentication seems to work as I get asked again providing wrong credentials. As soon as I enter my proper credentials for Zimbra the Mac OS X server seems trying to read my calendar from Zimbra - without ever finishing... (Getting events from server...).
Did someone ever try & succeed to read Zimbra calendars to be displayed in Mac OS X websites? Probably a certificate (self signed) issue?
Unfortunately I have not yet found a logfile which could tell me what is going on.