There are alot of debatable points in the argue of the Doomsday 2012. There's even a book on it. There's 1 i remember is the calculation by the Mayan people. They apparently calculated till that particular date and stopped. Which is on 2012 December.
And also, scientists/astrologists have argued about the Planet X that will come between Sun and Earth as mentioned above.
Others include meteorites, huge volcanoes, blah blah. But we will never know what will happen, until it happens, but it'll be too late though.
Anyways i think we should just live life as it is (to the fullest), and not care about when it will end, for you never know it might end tomorrow!