Many archeologists that specilize with Mayan heiroglyphics say that the end of their calendar aka "sun stone" was never intended to be an end of days tell, but it is marking the end of a cycle.
The reason the Mayan calendar does not continue on from 12/21/12 is some pretty advanced planetary vs science stuff that I really don't follow, but it seems to involve sun spots moving into retrogade..and that part holds true. There was a retrograde sunspot that scientists observed on 8/31/10 but it was so short that they decided to not even call it what it was.
Supposedly there are many planetary postions and cycles that the mayan calendar did predict, and modern day scientitst have been able to go back and verify. Despite being an ancient barbaric civ, their mathematial abilities were really unbelievable.
I don't buy that anything will happen 12/21.. but if you do a bit of astronomical research on things that have been changing.. we are undboutedly coming to an end of a cycle. Someone wrote an essay on why the mayans calendar could not move past 12/21/12 but I didnt read it.
I guess none of us will really know until that day comes... and we will get fun stuff like this texted to us in the meantime
9/11/01 Terrorism Act +
3/11/11 Japans Earthquake
(Like Savage mentioned.. Far more likely humans manifest some crazy crap)