IOS – I upgraded the software on the iPhone and the Yahoo Calendar stopped syncing via CalDav

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:

1. Open Settings on your iPhone, and tap Mail, Contacts, Calendars
2. Tap Add Account...
3. Tap Other
4. Under Calendars, tap Add CalDAV Account. Make sure you get that, not Add Subscribed Calendar
   
5. Fill in the server and account details, hit Next and Save.
6. Repeat with CardDAV instead of CalDav

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

I started encountering a similar problem with my Google for Business account last week. Mail.app was okay but Calendar refused to sync, returning the error message that "Your calendar URL is not a location that supports this request." every time Calendar tried to sync.

The solution was delete the account completely from System Preferences -> Mail, Contacts & Calendars. Generate a new one-time password for the account (I use two-factor authentication on all my Google accounts). And the re-add the account in that System Preferences pane.

All is well now after doing this.