Well, the Mesoamerican Long Count calendar just ends on December 21, 2012 when it reaches 13.0.0.0.0. That is based on the generally accepted idea that it began on August 11, 3114 BCE. There's nothing really out there that says definitively that the Mayans believed that the world would end on that day, just that the cycle would end and that a new cycle would start in it's place. Some people have interpreted that as it signaling the beginning of the so-called Age of Aquarius. There's of course all kinds of speculation and prophecies associated with that date, but I'm going with the idea that it's going to be a day like any other day.