First and foremost, December 2012 is too late for the world to end, as Britain will have already done an excelent job embarrising themselves at the Olympic Games.
On a brighter note, however, the world isn't going to end in 2012! I have no idea what a long count calendar is, but just because theres a number 13 doesn't mean Earth will implode. Does the planet destroy itself at one o'clock in the afternoon each day? Or on the 13th of each month? I'll answer that for you- no, it doesn't.
And theres a very good reason why the world won't end in 2012. There is this thing called the Laws of Physics, and what it basically means is that things don't just spontaneously happen, e.g. we don't all suddenly turn into cotton wool. Which is good, because cotton wool lacks the ability to do absolutely anything.
..