C|Net News has a very good article on the status of instant messaging (IM) compatability (Cries to open up IM have quieted). The bad news is that there seems to be little progress or incentives to make progress towards compatability any time soon. The good news is that AOL Instant Messenger seems to have stopped efforts at preventing the use of IM managers, such as Trillian (which allow users to interface with more than one IM system at a time). LawMeme has reported on some of the prior battles in this fight (AOL IM Battles Yet Another IM Service).
While progress in compatability may have stalled and consumers seem to be able to manage multiple accounts, this area remains a very hot battleground for dominance as new services, especially commercial ones, begin to be developed.