Can I sync my Yahoo calendar with my iPhone?
You can sync your Yahoo calendar with your iPhone over the air. These steps require iPhone OS 3.0 firmware or newer.
1. Go to "Settings" -> "Mail, Contacts, Calendars"
2. Touch "Add Account" -> "Other" -> "Add CalDAV Account"
3. Into the "Server" field type "yahoo" (caldav.calendar.yahoo.com will appear automatically)
4. Type in the Yahoo! username and password you use for your calendar
5. Touch "Next" and the iPhone will verify your username and password
6. Touch "Save"
Now when you open your iPhone Calendar you will see a page allowing you to select which Yahoo! Calendar layer you want to use.
NOTE: Once you sync your Yahoo! Calendar with your iPhone you will be upgraded to Yahoo! Calendar Beta automatically. You will not be able to go back to the old calendar version when this sync completes.