A good guess is that you have a handful of "non-UTF-8" characters mixed into your address book.

Here's the scoop: At Yahoo! (like many companies who offer address books), when you type-in contact information we convert all the characters into a UTF-8 code. However, if you add someone's contact information that's not in this UTF-8 form (which often happens when you cut-and-paste an international address from someone who's address book is non-UTF-8), then we can't read it. Hence, the data gets garbled.