I tried to see if anyone had this specific problem with the Pre, but couldn't find anything in the forums. My apologies if this is already a well-known issue:

I am syncing to my Zimbra account via ActiveExchange. It is absolutely seamless with Email and Contacts, but only ~2% of my Calendar entries show up on my Pre. The ones that do show up are editable - any Pre edits show up on the server pretty quickly, but new appointments do not appear to get transferred in either direction. I created a new calendar on the server, and that did not show up on my Pre either. I have seen people having isses with ActiveExchange systems that are not truly Microsoft, but I haven't read about this specific problem yet. Anyone out there have any ideas?

Details of Zimbra at my institution:
Client Version: 5.0.16_GA_2921.RHEL5_64
Client Release: 20090429021719
Build Date: 20090429-0227