I thought NASA's response for the particular "Mayan prediction" of the end of the world was so simple:
"Just as the calendar you have on your kitchen wall does not cease to exist after December 31, the Mayan calendar does not cease to exist on December 21, 2012. This date is the end of the Mayan long-count period but then -- just as your calendar begins again on January 1 -- another long-count period begins for the Mayan calendar."
Seems there were a string of end-of-the-world predictions this year:
http://www.nasa.gov/topics/earth/features/2012.html