The latest AIM beta for Windows from AOL contains "Encrypted IM".

Following the "more info" link, we find out some more tidbits about how this is to work.

It looks like the end-to-end pieces are part of AOL's move to offer enterprise AIM services on their existing infrastructure. Staying away from the Microsoft philosophy of having everybody build and maintain their own servers, AOL looks to be planning on using a common infrastructure and providing the security by encrypting data end-to-end.

Unfortunately, it appears that the security will only be available as part of an authenticated enterprise offering, so this will not be available to the general public and definitely not available for users who are anonymous.

However, there's a nice section in the "more info" area about how End-to-End encryption is essential in instant messaging and SSL is not sufficient to provide security for the users data because the AIM servers get to see all of the data unencrypted.