Yahoo changed their IM protocol or ports orsomething. If you are using Gaim or Fire, make sure you grab the latest version (Gaim 0.69, Fire 0.32.e) or you won’t be able to connect.

UPDATE: It seems that Gaim 0.69 still doesn’t do the trick. Thread here.

UPDATE #2: Fire’s suffering from the same problem Gaim is – the login fails with an invalid password message.