You're welcome, Brian. I believe that the Exchange<->Entourage interface is IMAP for email, and this weird WebDAV/xml combo for everything else. So "all" someone would have to do would be to solve the WebDAV/xml issue and Entourage would work pretty much just as well with Zimbra as with Exchange.
You'll still have the limitations of Entourage, though. Notably, it doesn't share its task list or notes with the server. I suppose it'd be possible to do something on the client machine (possibly using a combination of iSync, AppleScript, and/or iCal) to sync those with Zimbra.
Outlook doesn't work natively with CalDAV; the only reason it functions as more than an email client for Zimbra and certain other non-MS servers is that people went to the trouble of writing connectors for it. There is a group which is working on a generic CalDAV connector for Outlook:
The Open Outlook Connector Project
I'm not sure what you would give up relative to Entourage, but my testing suggests that Apple's Mail + iCal 3.0 could provide an effective combination for mail and group scheduling with Zimbra. You'd still have to do some things via Zimbra's web interface, but they're not things you're likely to need to do very often (like delegating calendars and server-based mail folders, and creating server-side filters). Mozilla Thunderbird+Lightning/Sunbird is another possibility; it isn't quite there yet, but we can expect progress since, unlike Microsoft, Mozilla doesn't have to be dragged kicking and screaming into supporting open standards.
And of course there's also Zimbra Desktop, but you need an Intel Mac and it currently has some fairly noticeable bugs (on top of not being a very Mac-like app). On the other hand, it offers pretty much all the goodness of the web interface.