You guys might want to take a look at JiveMessenger, the Open Source XMPP (Jabber) server from JiveSoftware. I've been using it for a while now, and it very stable. Now that I have Zimbra working great, I'm going to integrate JiveMessenger into the Zimbra webclient.
http://www.jivesoftware.org/messenger/
Some of the reasons I use...
*GPL License
*100% Java
*easy LDAP intrgration, only requires read only access & creates new accounts automatically.
*provides presence information via HTTP
*one of the most feature rich, see the following link
http://www.jabber.org/software/servers.shtml