After I hauled out Wireshark and watched what was happening on the server it was clear that it was doing an SRV request to try and find the CardDAV server for the domain I had entered in. Unfortunately the DNS management front-end that I normally use doesn't support SRV records, so I was all ready to switch my whole DNS infrastructure around (one of those 'round tuit' tasks that's been on my todo list for somewhat more than a year) until I wondered if the magic that is dnsmasq would be able to help me out.
It turns out that it can, and so after a little searching I'd managed to track down the magic syntax to do this:
srv-host=_caldav._tcp.home.mcmillan.net.nz.,davical.home.mcmillan.net.nz.,8008 srv-host=_carddav._tcp.home.mcmillan.net.nz.,davical.home.mcmillan.net.nz.,8008
Of course this is a special-case server for testing at home, and I run it on the funny port because it makes for less pollution when I sniff the traffic coming in.
In the real world you'd want to add SRV records for caldavs and carddavs, to get SSL for these services, something like:
srv-host=_caldavs._tcp.home.mcmillan.net.nz.,davical.home.mcmillan.net.nz.,443 srv-host=_carddavs._tcp.home.mcmillan.net.nz.,davical.home.mcmillan.net.nz.,443
Once I'd done that, and put in a few more hours of debugging, DAViCal was working with iCal's CardDAV, at least for viewing / editing / deleting contacts - there's still some work to do yet on searching. I've now also got it working with Evolution's 'WebDAV contacts' plugin as well, after a tip from Milan Crha on the Evolution IRC channel.
Heather gets home soon with the Apple phone I gave her when I upgraded to Android, so I'll hopefully be able to test against their new OS release and then get a new DAViCal release out next week.
Maybe. Hopefully :-)