...Or a gossipy secretary! I learned that one very early in my professional career. 
In reference to my previous post, the GPO setting for this is entitled "Warning Text" and the registry key is IMWarning
Code Snippet
IMWarning
REG_SZ
string (MaxLen 256)
Define custom text for Conversation window
Here's an example of what it would look like in the client's window. Again this is only for authenticted users (internal or external) and not visible on federated or PIC partner's windows.