A user brought to light a new feature of the 10.5.3 update.

With the release of Leopard OSX 10.5.3 Address book has got the option to sync with an Exchange-server. Screenshot: Mac OS X's Address Book Can Now Sync Google Contacts (Update: For iPhone Owners Only, UGH)

Can I use this synchronization with Zimbra? If so what do I need to enter at the Outlook Web Access Server field?

This appears to be an option for users who avoid using the iSync connector for managing their contacts.