I fail to see how anything short of an all-out nuclear war is going to cause the end of the world. The only countries that are crazy enough to use a nuke IMO are Iran, North Korea, and maybe Syria (if they got one). Even then, 1 nuke won't destroy the world. If anything, it would simply prompt an all-out multi-national (meaning 80% USA) invasion of the perpetrator, and as said perp would probably not have much of an arsenal, that would be the end of it,
Of course, there's always the chance that some virus off the northern shore of Lake Victoria will mutate and start turning people into zombies, Or that SETI willl pick up an Alien transmission, send a transmission in response, and inadvertently insult the aliens into annihilating us. Or that this post will somehow short out one of Anand's servers, which will set his new home theater on fire, which will spread to a nearby forest, which will spread into an uncontrollable wildfire, which will burn down the houses of CasioTech, SavoyBoy, Goku, and other morAns, which will in the process cause a surge that will feedback all the way to the power plant, which will shock some maintenance guy, which will send a surge all the way to NORAD, which will cause a temporary power outage, causing NORAD to switch to backup generators. Which will happen while the Stargate is on with an alien artifact in the Stargate's buffers, which will cause an overload that will take out the entire state of Colarado, which will ignite the Van Allen belt and boil the oceans, leading to the end of the world.
But hey, can't plan for everything.
Also, the Mayans predicted that the world would end in December of 2012. Should be interesting.