Zimbra provide an Exchange migration wizard which can be downloaded from the Zimbra Administrator website. This tool creates user accounts on the Zimbra server, if they do not already exist, and then imports email messages, attachments, calendars, and address books. Instructions: http://wiki.zimbra.com/wiki/Preparing_to_Use_the_Migration_Wizard
Exchange and Zimbra can be configured in a split domain configuration to allow for a staged migration of users from one platform to the other.
Zimbra can be configured to authenticate Zimbra users against Active Directory and can also share the Exchange Global Address List and interop with exchange Free Busy information to allow for side by side operation.
Zimbra version 8 supports mailbox and group synchronisation with Active Directory – automatically provisioning mailboxes when a new user appears in the Active Directory.