I am getting a fairly clear sense of how CalDAV clients and servers communicate with each other. I even read the spec! (Interesting aside, one of the primary authors of the CalDAV specification is Lisa Dusseault, who is a star character in "Dreaming in Code," the story of the Chandler project. She comes across as the smartest and sanest person on the team).
Here are the design questions I am currently mulling: