[Klug]

There's been some bugfix in 5.0.4 about iSync (not related to this "problem" but they should improve the user experience).

Bug 23602 - Alarm support in iSync per appointment
Bug 23423 - Creating a sub-calendar from ZCO does not get sync in iSync
Bug 24606 - Cannot upgrade isync connector from 5.0.2 to 5.0.3
Bug 24958 - folder filter doesn't work for attendees/organizers/recurrences
Bug 22390 - Will not sync appointment with changed calendar
Bug 24949 - iSync Connector doesn't handle malformed URLs
Bug 24698 - Custom email labels in AB can cause exception in contact sync
Bug 25102 - All calendars are not displayed in calendar section of Zimbra prefpane
Bug 25151 - Duplicate in iCal can cause exception

Using Entourage 2008 is another bunch of problems.