I've been reliving some of that cold war, 1980's, Red Dawn angst by watching Jericho on CBS, and I recently screened Wargames for my teenage son (who, quite to my surprise, enjoyed it immensely). So imagine my post-apocalyptic joy when Macworld mentioned a game (PC only for now, but available for the Mac soon) called DEFCON: Everybody Dies. It's the old Global Thermonuclear War game (which I sorely loved on my C64) on steroids. The downloadable demo has both single player and online multiplayer modes. I haven't fired it up yet only because I knew that if I did, this post would have been delayed...significantly.
Just remember, in a game where everybody loses, it can still be fun to try and lose the least.