Want to turn your iPhone into your personal assistant? "Those with iPhone 3GS or below, and run iOS4, can sync the appointments on your Yahoo! Calendar to calendar on your iPhone in a few easy steps. Once activated, you'll be able to add or find appointments with your iPhone, or if you're visiting Yahoo! Mail or Yahoo! Calendar on your PC," stated Yahoo!.
Here's what to do: If you're an existing user: Settings->Mail/Contacts/Calendar->Your Mail Account. Or If you're using Yahoo! Mail or Yahoo! Calendar first time: Settings->Mail/Contacts/Calendar->Add Account->Yahoo->Enter your account info and click next.
Note: "iPhone 4 will automatically activate Yahoo! Calendar once you sync your Yahoo! Mail. You can also sync your contacts to Yahoo! by using CardDAV protocol."