Short summary: I can't use evolution or sunbird to move zimbra-created events
from a zimbra CalDAV store to a google calendar.

For the following scenario, assume that I have two calendar stores, which
I am remotely accessing from two clients:
- a zimbra calendar and a google calendar
- both accessed via CalDAV from evolution and sunbird.
(Plus I can access the zimbra store using zimbra's web client.)

Let's assume that we can successfully create, modify and delete events
on both stores from both evolution & sunbird.

Now do the folloing:

1. Create an appt with evolution on the google cal.
   Create an appt with sunbird on the google cal.
   Note: we clearly have write access to the google calendar 
   from these clients.

2. Create an appt with zimbra's web client on a zimbra calendar store.
   Open the appt in evolution; change its calendar to the google
       calendar & save.
   What will happen: evolution will pop up an error window saying
       "Authentication failed"
   (If, instead, you use the "move appt" or "copy appt" operation,
   it just silently fails.)

3. Now create another appt in the zimbra store, but this time,
   create it using evolution, not the zimbra web client.
   As before, open the appt in evolution; change its calendar to the google
       calendar & save.
   What will happen: it works.

4. Now try all of the above, using Mozilla Sunbird instead of evolution.
   It comes out exactly the same (except that Sunbird fails less gracefully:
   It displays a pop-up saying "An error occurred when writing to the calendar
   <the google calendar>!" with "MODIFICATION_FAILED" as the error "number;"
   the event is then lost -- it no longer even appears on the zimbra calendar
   (oops).

Comments:

1. The fact that this fails in both evolution and sunbird makes me suspicious
of google's CalDAV service.

2. The fact that it fails only with events created using zimbra's web client
makes me suspicious of that client. I know, for example, that zimbra's web
client includes a ORGANIZER field in simple non-meeting appointment records, 
while evolution does not.
    -Olin