Sync Yahoo Contact with iPhone over the air (wirelessly)

Here is the official instructions from Yahoo:

If you are running iPhone 4.0 software or newer, you will be able to sync your Yahoo! Contacts to your iPhone or iPod Touch without plugging it into your computer. You can check your current version by going to Settings, then General. From there choose About, followed by Version on your iPhone or iPod Touch device.

To sync your account:

1. Go to your iPhone or iPod Touch settings application
2. Tap Mail, Contacts, Calendars
3. Tap Add Account
4. Tap Other and then tap Add CardDAV Account
5. For the Server you need to enter carddav.address.yahoo.com
6. Now type in the Yahoo! ID and password you use to sign in to your Yahoo! Contacts
7. Once you click save, your device will verify your Yahoo! ID and password
8. Next time you open up your iPhone or iPod Touch Contacts app you will see a page allowing you to see your Yahoo! Contacts sync into your device Contact list.
9. If you’d like to have the contacts you add onto your iPhone automatically sync to Yahoo!, you will need to go back into the device Settings application. Click on Mail, Contacts, Calendar. Drag the page down to the Contacts section and change the default Contacts account that you would like to save Contacts to.

Note: Some characters in the password may not work (such as ‘#’ and ‘ ‘) when setting up CardDAV. Depending on what version of the software you are using this may or may not be the case.

If you are having problems syncing your Contacts try going to settings->Safari->Clear Cookies. Then quit the Contacts application and restart it. If your phone supports multi-tasking make sure you completely quit the Contacts application.