Reddog Technology is now testing CardDav ...
True to our Open Standards Architecture ethos, we are now testing CardDav. CardDav is a next generation Open Standards contacts protocol which allows almost any device to connect, share and interact with contacts. We are extremely excited about this development within the Zimbra Collaboration Suite, because it completes the research and development for our Groupware (email, calendar and contacts) solution and allows our Zimbra infrastructure to scale within organistations without any need for connectors to make software work together.

Update.....

CardDav is now working but not in what we call the "Reddog Way". Apple Addressbook throws certificate errors when connecting to CardDav accounts. There is a bug in Apple OSX iCal, Zimbra and Apple are working on fixing this issue. A working CardDav solution should be completed within a couple of months.