The Open Source Applications Foundation are developing Cosmo, which is a CalDAV server written in Java and Chandler, which is a CalDAV capable mail and calendaring application written in Python.
Basic setup is as follows:
At version 0.7alpha3 the calendar is subscribed and displayed, but the 'displayname' property
which the server sends is not used and the calendar is displayed with a blank name. Double-click
on your new calendar and enter a name in the space available.
Chandler describes itself as 'an experimentally usable calendar', and it certainly feels that way. It also will only synchronise to the CalDAV server either when you press the "Sync" button or with a default frequency of hourly. This is quite different to the other clients I have used which all aggressively push new and changed items to the server as soon as possible, but which may be lazy about fetching updates.
Operation with RSCDS is not yet perfect but basic operation is satisfactory. I will be concentrating on making RSCDS interoperate with Chandler over coming releases.