Private Events and Tasks in User Calendars
Calendar Server 5.1 supports Private and Time and Date Only (confidential) events and tasks in user calendars. When creating a new event or task in Calendar Express, a user can specify whether the event or task is Public, Private, or Time and Date Only (confidential):
the release notes are just out now :
http://docs.iplanet.com/docs/manuals/calendar/ics51/relnotes/relnotes.html