I believe the jabber plugin is reversing the submitted hudson username and
password values (for password-protected hudson instances).

To workaround this issue, I've manually changed the
hudson.plugins.jabber.im.transport.JabberPublisher.xml file so that the
hudsonCiLogin element contains the password, and the hudsonCiPassword element
contains the username. This is seems to do the trick, but the manage hudson
form no longer contains a configuration block for the plugin.