The Diplomatic Pouch is the centerpiece of the Diplomacy hobby in North America. It is the brainchild and love-labor of Manus Hand, human extraordinaire, all-around good guy, and backstabbing pig-dog. Oops. Was that our out-loud voice? From the Pouch, you can find a plethora of articles on Diplomacy covering tactics, strategy, variants, and theory. Additionally, you can find games online, find news about upcoming events, and get in touch with local groups.
The North American Diplomacy Federation is dedicated to furthering the face-to-face hobby in North America. Its site is being renovated but includes a nice message forum. It also has pages and pages of incomprehensible numbers and statistics. Staring at them can help immensely in inducing a Zen-like trance.
Piggyback is our sister Diplomacy group in the Pacific Northwest. They host a number of excellent events, and have been known to come out and visit us far more often than good sense would seem to dictate. Oh, and most of their players are carebears who will actually make you feel good about being screwed.
Board Game Geeks has a wide array of information about Diplomacy, as well as any other board game you can think of - and quite a few that you really don't want to. They also have an extensive forum with conversations on all gaming-related topics, and will point you toward game stores online and near you.