NASA apparently doesn't believe in the Mayan prophesy that the world will cease to exist on December 21, 2012. They don't believe in it so much that they have actually taken time out of their busy schedule to produce a video to explain to you and me why the world will not actually be ending as predicted.
The video is titled "Why the World Didn't End Yesterday" and was set to be released on December 22, but NASA has decided to release it early, maybe as a sign they aren't so confident that they will be around on the following day to release it. The video goes over many reasons that the world won't be ending as "predicted."
However, it's likely that this still won't dispel most conspiracy theorists' theories on how the world is going to end. After all, the Mayans had access to much better technology than we do today, and would be able to predict our demise just about as well as they did theirs. Do you believe the world is ending on December 21, 2012? How will it happen?