Astronomers have located ~6000 asteroids large enough to wipe out all life on earth. They estimate there are over 20,000 asteroids of that size, and are constantly searching for more. Realistically, one could skip under the radar and not be noticed 'till it's too late...it's a big sky.
Solar flares have plagued our planet since its beginning. Now our technology is highly susceptible to them. There is a whole team of scientists dedicated to watching the sun 24/7 because we're entering another "solar maximum" which is a periods where solar flares are most probable; and they're estimating that the upcoming "season" could produce flares larger than what we usually experience...large enough to disrupt earth's magnetic field and allow the radiation to penetrate it, causing lots of problems.
Yellowstone Supervolcano, well, probably not a factor. But then again, humankind has never witness a supervolcano eruption, so nobody knows the warning signs.
So in reality, yes, the probability of life being wiped out in 2012 is fairly high. It has nothing to do with ancient calendars or prophetic writings. It has to do with the fact that earth has been through lots of shit, and will continue to go through lots of shit. We've just been lucky to have none of that shit in recent years. The date 2012 is just another doomsday date to scare people who enjoy being scared. The actual end of the world will happen when it happens.
PS. 1000's of people reach the end of their worlds every day (death). When you really stop to think about it, even if we do experience the final end of the world, the death from it won't be any different from the death we're already destined to face.