Just upgraded to 10.6.3 and immediately tested carddav.
Using only username (without @domain.tld, this is on the primary domain, so it works), password and server as
https://domain.tld:443, Address Book accepts the settings and generates no errors at all. Still my existing contacts don't show and added contacts are not present in Zimbra. So it seems we went from an obvious error to a silent one.
Has anybody had more luck? This would be a great feature if it worked.