@ajohnson: I'm sure there is definitely a precedent of this sort of behavior in postal diplomacy. I have read articles with anecdotes about people who have sent in forged orders etc, and the postal hobby has been going on for decades so I'm sure a lot of crazy things have happened.
However, whether or not such behavior is cheating really depends on the established code of conduct between the participants and game master.
The game of diplomacy may condone deception, betrayal, and manipulation, but as game, with defined rules and being a collaborative activity to have fun, it does not condone cheating.
In a game where some degree of espionage and sabotage could potentially make things more interesting and create a lot of fun, one has to clearly define where the line between acceptable behavior and cheating lies. It's up to the parties involved to set that line. If everyone agrees that a certain act is acceptable, then it's fine. However, it's tough to cover all possible scenarios and I feel that with any situation that's questionable, one should err on the safe side and not do it or ask the game master or other parties involved first.
I've heard of a general rule of thumb when it comes to defining what is cheating, the golden rule of diplomacy "You may not deceive the game master". In the context of this website, the game master is the website. Therefore, using someone else's account without their permission or controlling two accounts in the same game constitutes a deception of the game master and hence is cheating.