Looking through the bugs and found this one:
Bug 27380 - Leopard - CalDav only
Call me crazy, but I (and other colleagues) have had plenty of troubles with Leopard's iCal CalDAV support and Zimbra. We've all moved to iSync and it's been working wonderfully.
A couple of issues right off the bat:
"Resource Not Found" errors where it seems iCal tries to post to the root of our zimbra server ("/"). It also seems to not like calendars with spaces? That's more random, though.
I can't sync CalDAV calendars with iTunes. That means no syncing to iPods or iPhones like I do today. If it's supported by Apple it's buggy at best:
Apple - Support - Discussions - ical server to iphone sync ...
Have you guys found the magical iPod/iPhone <-> Mac <-> CalDAV <-> Zimbra solution? For us it's your iSync connector