If syncing your iPhone with Yahoo! Address Book doesn't work, make sure you are using iTunes 7.5 or later if you use Windows or Mac OS X 10.4.11 or later if you use a Mac. Using the latest version of iTunes for Windows or the latest version of Mac OS X should resolve this.
Note that this could also be caused by an invalid email address present in your contact list. Valid email addresses are of the form username@domainname, e.g. steve@mac.com or user@example.org.
If you use Windows, check your iPhone for invalid or improperly formatted email addresses. If you use a Mac, check your iPhone, Address Book (or any other contacts syncing application, such as Entourage) for invalid or improperly formatted email addresses.
If you find any, either correct them, or remove them.
Important: Information about products not manufactured by Apple is provided for information purposes only, and does not constitute Apple's recommendation or endorsement. Please contact the vendor for additional information.