If you are running iPhone 3.0 software or newer, syncing calendar to your iPhone or iPod Touch can be done without plugging into your computer. You can check your current version by going to Settings > General > About > Version on your iPhone or iPod Touch device.

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 CalDAV Account".
5. For the "Server" you can simply type "yahoo"
6. Now type in the Yahoo! user name and password you use to sign in to your calendar
7. Once you click save, the iPhone or iPod Touch will verify your username and password.  
8. Next time you open up your iPhone or iPod Touch Calendar you will see a page allowing you to select which Yahoo! Calendar layer you want to view or edit.

NOTE: Once you start syncing your 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 product once this is complete.