Life Imitates ARG, Unfortunately
One of the leading figures indicted in the March 11, 2004, train bombings in Madrid used a simple trick that allowed him to communicate with his confederates on ordinary e- mail accounts but avoided government detection, according to the judge investigating the case. I sure have mixed emotions about this one. Terrorists all had the password to a web-based email account, where messages were saved as drafts, thereby never being sent across the wire. Government had no means to monitor it whatsoever.
Those familiar with ARGs know that this technique has been employed on numerous occasions, albeit with non-functional web-mail applications (so players couldn't go in and delete/add/edit messages of their own), to give players access to game characters' email accounts and the messages they contain.