Synchronizing the Calendar
1.Open the settings application.
2.Select Mail, Contacts, Calendars.
3.Select Add Account.
4.Select Other as account type.
5.Select Add CalDAV account.
6.For server, type ADDRESS/owncloud/remote.php/caldav/principals/username
7.Enter your user name and password.
8.Select Next.
9.If your server does not support SSL, a warning will be displayed. Select Continue.
10.If the iPhone is unable to verify the account information perform the following:
•Select OK.
•Select advanced settings.
•Make sure Use SSL is set to OFF.
•Change port to 80.
•Go back to account information and hit Save.
Your calendar should now be visible in the Calendar application
tratto da: http://owncloud.org/support/synchronisation/ios/