:excited:.. finally found it. One can use an iCloud Account for syncing caldav and carddav.
I tried this to sync my Yahoo! Calendar, and couldn't get it to work. Could you explain what you did?
I've created a new iCloud account, renamed it to Yahoo, turned off syncing Contacts, and changed the CalDav server to caldav.calendar.yahoo.com. What is interesting is that the WP Calendar app settings shows the Yahoo account, and also shows the named Calendar I have defined under that account. And I can create new calendar entries that are saved to that account, but once the sync action finishes, they vanish from the WP app, however they are posted correctly to Yahoo, you can see it in the web calendar interface, even syncs to my iPhone.
The whole need to use the iCloud option is stupid in the first place, why there isn't an option to set up a CalDav account under Advanced Setup makes no sense.