Two-way CalDAV and CardDAV sync on iOS?
CalDAV and CardDAV should definitely be two-way on iOS 4. I'm doing CalDAV right now through Google and I can confirm it works fine. I haven't tried CardDAV, but I haven't heard of any other issues. It's possible that your CalDAV server isn't set up properly, or you "Subscribed" to the CalDAV server instead of adding an account. Hard to say without knowing the particulars of your setup.

To add a CalDAV account:


Open Settings on your iPhone, and tap Mail, Contacts, Calendars
Tap Add Account...
Tap Other
Under Calendars, tap Add CalDAV Account. Make sure you get that, not Add Subscribed Calendar

Fill in the server and account details, hit Next and Save.
Repeat with CardDAV instead of CalDav


You should be all set. If it doesn't work, check out things server-side.