The Mayans' calendar was a "Long Count" calendar. It was based off of arethmetic. It only could go so far, because their math could only go so far. It just so happens that, on that day, they need to reset it. They called it the end of the "Great Cycle". The Mayans did NOT prophecise Doomsday. They actually planned to have a hug-ass celebration. So some paranoid idiot said "Oh noes, they sayed the world iz gong 2 end!!11!1one" and so everyone starts freaking out.
The government/scientists/whatever wouldn't let this leak. Everyone would stop working and stuff and the whole world would just stop, waiting for it to "end". So unless every scientist at NASA is completely wrong, I don't think we have much to worry about.
For your amusement:
http://www.newgrounds.com/portal/view/517835(No I did not make it, but you'll still stuff bricks.)