iChat, the default AIM client on Mac OS X, allows Apple .Mac users to encrypt chat if both users are using .Mac accounts. The encryption certificate for the encrypted chat is created on the users machine.
When attempting use of certificates that have been stored inside a Keychain, the certifications are linked to the CPU that they were created on. In this situation unlocking the Keychains master password WILL NOT allow encrypted chat between computers if the Keychain has been moved off the primary computer that certification was created on to another computer for data discovery or live surveillance. At this time no workaround has been found.