I am considering setting up a Citadel groupware server. It natively supports syncing via the GroupDAV protocol.
Is there an app for the iPhone that lets it sync its calendars, contacts, and tasks with a GroupDAV server?
As far as I can tell, GroupDAV is a superset of, or at least closely related to, WebDAV/CalDAV, for which the iPhone has built in support.
You could try fiddling with your iPhone's calendar app. Adding a CalDAV account may work.