Interesting. From the government's copyright website:
"Copyright does not protect the idea for a game, its name or title, or the method or methods for playing it. Nor does copyright protect any idea, system, method, device, or trademark material involved in developing, merchandising, or playing a game. Once a game has been made public, nothing in the copyright law prevents others from developing another game based on similar principles. Copyright protects only the particular manner of an author’s expression in literary, artistic, or musical form."
So it would appear you can make a Diplomacy site, call it Diplomacy, use all the same rules, and as long as you produce your own maps and other imagery, you're good to go.
http://www.copyright.gov/fls/fl108.html