I just installed 10.6 on my home machine and I'm happy to report that it improves Zimbra support, most noticeably in providing seamless autocompletion for attendees in iCal. I also see hooks for CardDAV, which should be very useful once
Bug 22008 - WebDAV: Add support for CardDav is finished.
In short, the argument for Entourage is correspondingly weakened as Mail/iCal improve. However I'd make the following suggestion: if Zimbra offers Entourage support, it should be part of Zimbra Professional to reflect the additional development costs associated with supporting a proprietary protocol, similar to Outlook support.
Meanwhile if CardDAV works out with Zimbra 6 and Snow Leopard, there's going to be very little need for the iSync connector--I'd think you'd only really want it for non-Intel Macs that can't use Snow Leopard. This means that many shops with Macs may be able to reduce their Pro licenses as long as they use Mail/iCal, or other standards-compliant clients. (E.g. Thunderbird/Lightning.)
One thing that Mac Office users might have to look forward to is
Outlook for Mac. If this turns out to be more Outlook-like, then it might give Zimbra more options on ways to support it. (E.g., Outlook's "internet free-busy" feature is exploited by Zimbra to provide FB publishing/lookup.)
However if you're considering using Entourage in a business environment, you should be aware of the challenges it raises for effective workstation backup. Entourage stores
everything in a single database, and backing that up while it's open is likely to result in a useless backup. I suppose that's okay as long as the data is also stored on the server, but information stored in local folders, including mail and addresses, is at risk. The same may apply to Outlook for Mac, too.