Yesterday i wanted to add a buddy using a regular and open jabber service to my Google talk account provided by Google Apps.
In order to be able to communicate with external jabber servers (e.g. jabber.ccc.de) from your Google Talk Account you need to set up service records (SRV) in your DNS configuration for your domain. Google has a howto on this over here. You need to configure 5 new records with different priorities on port 5269 pointing to various google xmpp servers.
Example picture from one new DNS record in my domainfactory configuration interface:
A dig on your _xmpp-server._tcp.Domain.org should look like this example after configuration:
bobafett:/# dig srv _xmpp-server._tcp.andreas-lehr.com
; <<>> DiG 9.7.3 <<>> srv _xmpp-server._tcp.andreas-lehr.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 1024
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 2, ADDITIONAL: 2
;; QUESTION SECTION:
;_xmpp-server._tcp.andreas-lehr.com. IN SRV
;; ANSWER SECTION:
_xmpp-server._tcp.andreas-lehr.com. 3600 IN SRV 20 0 5269 xmpp-server2.l.google.com.
_xmpp-server._tcp.andreas-lehr.com. 3600 IN SRV 20 0 5269 xmpp-server3.l.google.com.
_xmpp-server._tcp.andreas-lehr.com. 3600 IN SRV 20 0 5269 xmpp-server4.l.google.com.
_xmpp-server._tcp.andreas-lehr.com. 3600 IN SRV 5 0 5269 xmpp-server.l.google.com.
_xmpp-server._tcp.andreas-lehr.com. 3600 IN SRV 20 0 5269 xmpp-server1.l.google.com.
..
After this you should be able to add buddies from other jabber servers in your google talk/app account.