[LinuxBender]

That is related to the last paragraph, in that one has to trust the application or other applications with access to your chat history cache can not read your keys and know how to use them. On a cell phone, I would say that is quite a challenge, given that applications can be updated by other parties. In my opinion, unless you fully control the OS and applications and no application can update itself without you explicitly taking action, then one can not say for sure that the message was protected.