Sry not a Mac user, but just wanted to check if anyone here (still having issues with ZimbraHelper that is) after trying all the suggestions ever filed a bug / support ticket / got any good logs from /Users/<user>/Library/Logs/Zimbra/com.zimbra.XXXXXXXXXX/sync.X.log (if it was churning hard on a particular sync or something).
Not just for iSync, but there's a nice collection of tips here:
iCal_Issues - Ajcody-Notes - Zimbra :: Wiki