Both ZWC and Outlook via ZCO support (adding/reading) attachments as part of an appointment. I just noticed that iCal doesn't allow this for events on a Zimbra CalDAV calendar, although it does for a calendar located "on my computer".
iCal claims it has this ability (iCal 3.0.6 help, "Attaching a file to an event invitation", specifically mentioning CalDAV). And the CalDAV RFC mentions the capability, too:
RFC 4791 - Calendaring Extensions to WebDAV (CalDAV), section 8.5
So is this a limitation of Zimbra CalDAV, or does iCAL actually depend on some funky stuff that's only in iCal Server?