Affirmative.  We have broard experience in integrating Zimbra with directory services and other authentication services.

Enterprise services: Not only does Zimbra itself integrate with LDAP for authentication, but also we have in-house expertise with custom scripting and programming to perform, for example, auto-account creation/provisioning/pruning between Active or Open Directory services and Zimbra.  Other examples include distribution list synchronization.

Shared hosting: Want to be able to lookup your domain's email addresses from within your mail client, for example, Apple's Mail?  We've also used this experience to customize the 01.com shared Zimbra hosting solution, so that users are limited by domain, and using their email addresses and passwords may securely (SSL) access their Global Address List (GAL), which includes all addresses within their domain and sub-domains, from their mail clients (AddressBook, Outlook, etc.) across the internet.  Please see the private Knowledgebase once you sign up for complete instructions and documentation on LDAP configuration.