OLPC Chicago Jabber Server is jabber.olpchacks.org.
To setup your XO, open the terminal activity and type:
% sugar-control-panel -s jabber jabber.olpchacks.org
Restart Sugar/X (ctrl-alt-erase) and you will connect to the jabber server.
The OLPC Chicago community Jabber Server was created by Harper Reed on March 1st based on a request from the OLPC Chicago mailing list. It is currently running ejabberd on debian etch and hosted at amazon's EC2. The installation was based on a write up by Morgan Collett (here).