Many venerable and excellent games such as Metamorphoses: Alpha, Gamma World, the Morrow Project, and even Twilight: 2000 are set in a post-apocalypic world. So, too, television shows and films such as Arc II, Logan's Run, or Earth II. By definition, these all posit a devastating cataclysm of some sort, usually a war or man-made ecological catastrophe.
In Transhumanist terms, such a devastating war would be termed a singularity. Singularity in this context is a term coined by science fiction author Vernor Vinge, to describe an event in the future beyond which it is impossible to predict anything about society, history, technology, etc. Usually, the term is used in conjunction with the development of artificial intelligence, nanoassembler technology, etc., but a nuclear war would fulfill the definition just as well.
These are what are known as "hard takeoff" singularities. They happen suddenly and jarringly, with little or no warning. You don't know it's coming until you see the radar tracks of the missiles arcing over the polar ice cap, or until the supercomputer announces that "Action will be taken" unless its demands are met...
There is another sort of singularity, however, that I think would make for a spiffy campaign setting for this sort of game. This is the "soft takeoff" singularity. This is the singularity that takes its time coming; it may even be consciously planned as a singularity, but need not be. And it might not necessarily be a move "up".
But the world they describe is not the result of a war. Rather, it's the tail end of a long, slow decline in human civilization. And what brought about this decline from an apex of technological grandeur? (Highlight to reveal invisotext spoilers) All the best and brightest people left Earth for the stars, and are now only remembered in half-mythological terms as "Lords". Those who were left behind, the second-rate folks who didn't have the drive to innovate and create, gradually lost their knowledge as a result of laziness and complacency as much as anything else. I think this would make an awesome background for a campaign, perhaps focused on restoring human civilization to something approaching its zenith.
Just a thought for those who like post-apocalypic games, but might want to shake up the nature of the apocalypse. I'm thinking Gama World with large, functioning cities, a transportation infrastructure, and the like. Could be neat!