Since Exchange 2010 provides WAN replication natively, Zimbra having to rely on SAN-SAN replication or VMware Fault Tolerance for supported WAN replication puts Zimbra at a severe competitive disadvantage in a number of cases.
In our experience, many mid-sized (e.g. 500-5,000 mailboxes) prospective Zimbra customers do not have a budget for a pair of SANs nor are they going to deploy an enterprise-grade VMware environment for just email/groupware.
So, although Exchange's server hardware requirements and licensing costs exceed Zimbra's, when you add in the cost of VMware and/or a pair of SANs to get WAN replication, Zimbra now becomes the more expensive solution.
Whether WAN replication is best accomplished as originally planned for ZCS 4.5, via log shipping as in the latest RFE or via some other method is a discussion for another day.
Our view however is that having supported WAN replication baked-in natively in Zimbra would greatly improve Zimbra sales in the mid-market.
All the best,