On Saturday 24th September Yahoo upgraded their messenger service, meaning (afaik) that all users had to download the lastest client from the Y! Messenger website. I use Y! IM, but through Jabber. The upgrade broke the transport. There’s now a fix to the transport to work with the new authentication scheme.
The updated version is only in CVS at the moment, but with any luck this means the slow reconnection of Jabber users to their Yahoo accounts.