Back in April I was involved in an internal thread about the impact of the iSync to CalDAV switch on 10.5 and it was mentioned that support/customers would get a detailed explanation of the situation. I couldn't find anything relevant in the release notes. I've asked the developers for a follow-up on that thread. I will let everyone know what I get back and I'll also update my wiki page on Mac Issues. [ Ajcody-Notes - Zimbra :: Wiki ] I'll take the fault for this since I'm the one that raised the issue and didn't confirm it was done; sorry.
For the brave, you could always use connectors before the 5.0.6 release (on MacOS 10.5) . Since this would end in an unsupported situation [support staff and developers would tell a customer in a "support case" to upgrade to a new release] it would be best to encourage your users to keep a very clean and simple calendar as to avoid needed support. This might buy some time until the bugs mention above are resolved.