Long time listener, first time caller.
To summarize, it seems Zimbra is negatively affecting iCal events sync'ed using .Mac between two Macs. Without luck I've exhausted my options to solve this issue on the Apple website iCal discussion forums.
Syncing calendars between my MacBook Pro, .Mac and my wife's MacBook has been on the fritz since at work we recently migrated from not having a groupware solution to Zimbra.
I think my problem has something to do with the entries here:
OS X Leopard 10.5.2 > iCal > Preferences > Accounts >
That's where Zimbra puts my work CalDav account. My Zimbra calendars appear in their own Group in the sources pane of iCal. I tried deleting the Zimbra account to see what happens but it looks like Zimbra puts its account right back in there at the time of my next .Mac Sync. I'm not sure how that part works. So it appears as if there might be a conflict between iCal Groups and .Mac syncing...