AOL Instant Messenger is a client-only lock-in that will be undermined totally once the Jabber protocol (or some other IM protocol) ubiquitizes into the same grade of Internet infrastructure as SMTP and POP3 provide for mail service and HTTP provides for Web service.
I agree that it is inevitable that instant messaging protocols will eventually be opened. It would be the best for AOL to open up all their protocols as it would give them an immediate advantage and help them become the standard.
Opening just the AOL client protocol is missing the point (and it has already been mostly reverse-engineered as well as licensed twice). Much more interesting and useful is to open the protocol that AOL uses to communicate between servers. To be able to bridge instant messaging systems at the server level opens up a whole world of possibilities. It would allow variation in clients while still supporting interoperability. It would allow differences in protocol that might not be included in the standard, such as different encryption formats or additional features. An extensible protocol could allow some of this, although adding new data encryption techniques after the fact while maintaining interoperability would likely be difficult or impossible. Opening the server protocol would also allow more secure, inside-the-firewall servers that have their own unique features and configurations, such as message logging (a legal requirement in some industries).