As the project manager for Bedework (http://bedework.org), we are always looking for new CalDAV clients to test against. I was able to display Bedework calendars from Spicebird 7 and 7.1 (but only when the Spicebird cache is enabled).
However, as there is no place to present or save authentication information, there is no way to write to CalDAV calendars or to schedule.
Are there plans to accommodate CalDAV authentication credentials in the near future?
Sorry for the delayed response.
Spicebird 0.7.1 does support CalDAV calendar with authentication and modifying events. While creating the calendar you need to select CalDAV as calendar type and provide the URL. The auth credentials are prompted as soon as the calendar is added (and retained as long as SP is running). You can save the credentials by checking the option in the prompt dialog itself.