[link|http://www.opengroupware.org|http://www.opengroupware.org]
Not quite distributed calendaring, but a good step toward an open source Exchange replacement.
Mission: To create, as a community, the leading open source groupware server to integrate with the leading open source office suite products and all the leading groupware clients running across all major platforms, and to provide access to all functionality and data through open XML-based interfaces and APIs.
Haven't tried it myself, yet.