Thanks all for the replies.
mvalcarcel - I can use this to launch Zimbra with the "Open Calendar" button, but doesn't tie the calendar into gnome the way it is with Evolution.
Tecton - This is a clever work around, but still relies on Evolution's integration with the calendar.
I'm looking to do something like this: http://www.webupd8.org/2011/09/googl...tegration.html
but with Zimbra. From looking at the script this doesn't seem to be a simple process, so if one isn't already available, I'll have to explore other options.