A new open-source CalDAV server is making progress: DAViCAL now implements CalConnect's Freebusy Read URL. Written by New Zealander Andrew McMillan, DAViCAL "is a project...to create a straightforward CalDAV server for shared groupware calendaring. The project is written in PHP and uses a PostgreSQL database for backend storage."
Another opportunity for someone to write a cookbook to allow mere mortals to install and operate a low-cost CalDAV-compatible calendar-sharing server at home or elsewhere. Any takers? I may get around to running it and writing it myself.