- Add appropriate SRV records to DNS,
- Punch a hole in the firewall,
- Add the domain to the service configuration,
- Uncomment the S2S lines in /etc/jabber/jabber.xml,
- Start the service.
That was it! Since all the users are already in the directory and the iChat server is tied into the directory, everyone has immediate access.
The thing with the s2s stuff was kind of ridiculous. Without going into the jabber.xml file, the iChat server just runs the client port allowing only directly connected clients to chat. The server admin UI did not include an option to enable the server side connections such that inter-domain chats can take place. But it was fairly simple to enable so not a big deal really.
In summary: XMPP is good for you!