I'm sure everybody knows the whole 2012 doomsday theory. I, personally, don't believe the world will end on 2012, but I have put a little thought into how the world would end if it did. Now, the first thing that comes to mind when I hear someone talk about the world ending in 2012 is Sarah Palin. 2012 is the year Sarah Palin runs for president. She were to win...head for the hills. We're all doomed. I'd sooner assassinate her myself than wait for her term to run through. The second thing I think of is the Y2K incident, which is, basically, identical to the 2012 thing: the world will end on this specific day because of something stupid. Nuclear warheads will all shoot at the same time, bombarding the planet with explosions and what not. Those who survive the initial impact will, then, die due to the radiation. Lastly, i think of an asteroid impact. Like the one that, supposedly, wiped out the dinosaurs. To see what this would like, watch Deep Impact or something.
P.S. The Mayan calendar is, basically, the only basis of the theory. The thing had to end eventually. You can't expect the thing to go on forever.