Using Google Talk with a Google Apps for Domains Address
1. Add the account as you would any other jabber account, using account add jabber firstname.lastname@example.org password.
2. Do account list to get the number of the account, then do account # set server talk.google.com
Note: If you're using libpurple, you may need to use account # set connect_server talk.google.com.
3. Do account # on and you should be good to go.
Do not forget to set up DNS for your domain as per https://www.google.com/support/a/bin/answer.py?answer=34143.