Don't count on the AIM interoperability to be 100%. "Fortress AOL" tries to do everything they can to lock out the competition.
see this article:
[url]http://www.cnn.com/2002/TECH/internet/02/26/trillian.restores.im.idg/index.html[/url]
IRC is old school, and is by far the oldest of the bunch. Was often used for swapping mp3s and warez before napster/morpheus. Still is a haven for warez, pr0n, and 133T h@x0r5 (script kiddies). Undernet (a particular IRC branch) used to be pretty big, but I've heard that various problems are bringing it to its knees.
ICQ used to be the AIM alternative, for those who despise AOL. Tired of having competition, AOL bought ICQ. Despite being owned by AOL, it has never been interoperable with AIM, but tends to be more interoperable with other clients. Also supports more plugins like encryption (for secure chat).
Throughout this Trillian thing Microsoft and Yahoo have played it fairly cool (surprisingly). Probably don't want to come off like hypocrites for blocking others when they've been crying for access to the AIM users forever.
Viper Out