InternetWeek [internetweek.com] story says it will be compat with AOL and Yahoo.
Within the next 90 days, Microsoft plans to release its much-anticipated Office Communicator 2005 client and a service pack for Live Communications Server 2005 that will allow interoperability with MSN, AOL and Yahoo, the three major public instant-messaging networks.
This one sounds like it might be a bit more advanced than the Trillian and GAIM crowd. However, to get the most benefit from it you may have to purchase the server and run it yourself. I'm getting the feeling that this is a corporate IM solution rather than a replacement of the consumer IM client.