from http://kopete.kde.org/
01/11/2004 - The Great Yahoo Block of 2004
The Yahoo Messenger servers are blocking clients who can't handle the new authentication method that was unleashed on all of us on Janurary 9th. We're working on a fix, but it will take some time. If you want to monitor the progress of the fix, please follow bug 72219 for info
This bug says it repeatably asks for a password, whereas it just hangs for me...