Darwin Calendar Server
The project I've been working on at Apple is now announced, and it's the Darwin Calendar Server. This server is a CalDAV implementation written in Python using the Twisted Framework.
Since we annouced this last night, the poor little G4 we had running the Subversion and Trac instances has gotten rather intimited by all the traffic, and at the moment it's taking a nap. So if you are willing to wait until next week to take a look, you'll probably have better luck then. (Yes, new hardware has been ordered.)