Spoiler Warning: This entry contains a few spoilers. If you haven't seen This is the End, you may want to skip it.
I watched the movie This is the End last night, and it prompted some thoughts on campaign possibilities. There is a general convention that players sign-on to the campaign conceit before play. If the group sits down for a game of Call of Cthulu, it has been established that everyone wants to play a horror game. Before a session of Star Wars, everyone's basically agreed to play space opera. If you shift gears on your players during the campaign and transport your Call of Cthulu players to Narnia, they might cry foul (and with good reason). But there is a place for judicious use of surprise campaigns.
I started GMing with Ravenloft and while you can run it as its own campaign setting, where the players start as natives, it was originally intended as a surprise to drop in an existing game. For example you might have a party adventuring in Faerun and when one of the player characters commits an evil act, the mists appear and take them to Ravenloft. I've been thinking more and more about springing surprise campaigns like this on players. While it can make for more mystery and excitement it can also tick off the group if you aren't careful (or if it just isn't the kind of thing they enjoy) so know this sort of tactic isn't without risk.
In the past I've dropped horror campaigns on my party. Mostly this was by bringing characters to Ravenloft in a standard campaign, but I've also thrown zombies at players in modern games. I contemplated turning my Crime Network campaign into horror at one point as well, having one of their fallen PCs come back as a spirit bent on revenge, but opted against it. Now I'm thinking of how you might drop an apocalypse on a party. I haven't tried this yet, so this well could crash and burn. Watching This is the End, gave me some ideas though.
I think the crucial thing is it needs to happen in an ongoing campaign where the players are invested. It would probably work best in a modern mundane setting because players won't expect it, but could work in anything (from alternate history to fantasy). That time investment, in my experience, really makes a difference. With Ravenloft it is one thing to make new characters and then pull them into the mists first thing, quite another to have it happen well into an ongoing campaign.
In terms of venue, the apocalypse could be anything from the biblical Apocalypse to nuclear war, zombies, etc.
One thing the movie shows is that the biblical apocalypse could provide ample room for adventure following the rapture. The rapture is the moment when all the good people (in the bible it is all the Christians, but the movie conceit that it is all the good people probably works better for an RPG) ascend to heaven. There are different ideas about how this is all supposed to play out, but one line of thoughts believes there is a period of tribulation where those who didn't ascend are left to suffer on earth as the apocalypse unfolds. This is essentially the premise of This is the End and I believe this works great for a few reasons.*
You can extend this period for as long as you want. There is all kinds of wiggle room to decide how things play out and exactly what sort of world the characters inhabit during the great tribulation.
This forces the players to go into survival mode but with characters they're invested in. If you begin the campaign at the moment of the apocalypse, the players don't really have much to lose except characters they rolled up two minutes ago. If you drop it into a six month old campaign, their survival matters a lot more.
My favorite reason is the bad people. After the rapture, the only people who are left are the bad folks (you can decide for yourself exactly what this includes, in the film it was everyone from the vaguely snobbish and ungrateful to the truly murderous). What is hell for the players, is heaven for the GM. This is an environment dominated by tribes of cannibals, violent motorcycle gangs and even a few opportunistic politicians.
Of course, you might not want to use the biblical apocalypse. There are plenty of other options here: zombies, plagues, the Islamic Yawm ad-din (day of judgment), the end of Kali Yuga, Ragnarok, etc. You can even mix and match if you want. There is no rule that it has to follow one tradition or genre. You can get as creative as you like.
I would just mind the risk here. This is the sort of thing where you have to be willing to pull the plug if it bombs with the groups.
*I'm no theologian, so I am sure I am oversimplifying here.