Okay I think I have a fix
First, REMOVE the Zimbra iSync from Preferences (right click in Prefs and remove)
Second, REBOOT computer
Third, download latest Zimbra iSync (try this one
zimbra-isync-4.5.9_GA_1450.MACOSX_UB.dmg - Powered By Kayako eSupport)
Foruth, install Zimbra iSync
Now, if you want to sync with the iPhone you have two options
* sync via iTunes (phone will only be up-to-date when plugged in)
* sync via MobileMe (costs $$$ - but supports push)
I'm using MobileMe for now - during freel trial and I can confirm that items added to the phone, show up in Zimbra, and vice versa.
BUT YOU HAVE TO UPDATE your version of Zimbra iSync - it's THIS that causes all the issues!!!