Yeah, ran into the same problem a few minutes ago. Haven't found a solution yet but if I do I'll update.
Edit: This might explain our woes:
[SOLVED] meetings created in iCal 3 CalDAV don't generate invitations
Seems to be related to an issue w/OS X 10.5.7 so either an OS X downgrade or a Zimbra upgrade is required to fix this particular issue. #lame