What seems to have happened is that Yahoo has decided to make IM chat available from Yahoo e-mail. You no longer have in install a separate Yahoo Messenger program which may not even work properly anyways. Now you can even chat while you're trying to compose and send out an e-mail.
Actually, the problem with this version of chat is that it takes every single entry in your Yahoo Address Book and puts it into your chat list. Even if an address was entered into your address book simply to prevent e-mail from the sender from being accidentally put into the spam folder, Yahoo has decided that the address should be in your IM list.
What a wonderful bit of progress!