I was comparing Zimbra and Exchange, with a userbase including a fair percentage of Mac people, so I followed developments on all three fronts for a while. I don't have any inside information, but I expect that Snow Leopard will use Exchange Web Services if it doesn't use ActiveSync. EWS is also what the next version of Entourage, which has been in beta for a few months, will use.
As for Zimbra, Snow Leopard will offer improvements since Zimbra 5.0.17 + Snow Leopard will support Calendar Delegation and proper attendee lookups/autocompletion. See
Bug 26619 - Include necessary LDAP attributes to provide ical oattendee lookups Bug 21489 - CalDAV: Add support for calendar delegation Bug 38350 - Need to add preference to enable iCal style delegation
Somehow I suspect that the fix for
Bug 21490 - CalDAV: Add support for availability might also ultimately require Snow Leopard, but that's just a guess based on intuition.