As of iPhone OS 3, the iPhone and iPad are able to directly access CalDAV calendars on Calendar Server. This is achieved by simply setting up a CalDAV account via the "Mail, Contacts, Calendars" tab in the "Settings" app. Choose "Add Account...", then "Other...", then "Add CalDAV Account...". Enter the server hostname, user id and password, then tap "Next". The iPhone/iPad should then attempt to connect and verify the server details. If all works OK, a new account is created, and the Calendar application will be populated with that account and its calendars. You can then create, edit, delete events on the Calendar Server.