A while ago, I mentioned that it would be nice to make the encryption that is available in AIM for .Mac customers compatible with other AIM accounts. The Off-the-Record Messaging program provides a proxy that enables this. This proxy essentially provides all of the key management and the encryption support needed.
Unfortunately, it is not really integrated well with iChat or Proteus. You need to use the proxy to accept the public encryption keys from the people that you are chatting with. There is a plugin for gaim, and there is built-in support in Adium X, so the interface should be better in those applications.