6.0.2 is out with support for CardDAV which I know Mac/Zimbra people are waiting eagerly for so they can start syncing contacts between Address Book.app and their Zimbra servers. However, the bugzilla notes contain the following statement from Jong Yoon Lee:

The current version of Snow Leopard Address Book app does not work on any CardDAV server other than Apple's. Zimbra will have CardDAV and Address Book app support by 6.0.2. Apple will release a patch for Address Book app that supports Zimbra and other 3rd party CardDAV server, but we don't know when.
I can confirm that OSX 10.6.1 and ZCS 6.0.2 don't talk - Address Book.app can't add my Zimbra account. Is there any further information or rumour on when Apple might do their end of the bargain? ..can I ask how you've tested CardDAV support (e.g. is there anything user-hackable on the OSX end that makes this work?)