Likewise, we're also suffering from the same problem. Appointments created on the Zimbra server will sync with Outlook, but appointments created in Outlook will not sync with the Zimbra server.
It's be really nice to get an updated Outlook connector, but I suspect this bug is quite far down the list. See
list
Outlook 2003 SP3
Zimbra Server 5.0.8_GA_2458
Zimbra Connector for Outlook 5.0.2662.8