While in the past instant messenger networks from AOL, Yahoo, MSN and others were complete separated from each other, this is currently changing. While some clients like Trillian tried to connect to multiple networks from one client, we are currently seeing the networks to connect to each other in the background (server to server).
IBM will ship Sametime 7.5 with support for AOL, Yahoo and Google. Yahoo Messenger is currently testing a Beta to connect to MSN. Skype is not in the news to connect to anybody else, they still use a proprietary protocol while the rest of the world moves to SIP.