Take a look at this:
http://www.microsoft.com/mac/itpros/entourage-ews.mspx
It was announced during MacWorld.
Granted, this is just a beta at this time, and as such there are obviously no guarantees it will work with Zimbra at all. But the prospects are good. However, over time, my guess is that it probably will. I haven't tested it myself yet, but will as soon as I have time.
Report back if you try it.