I love how human beings are so self righteous that we believe we will be the end of the world. Hey, guess what? The world will do just fine once we're gone. On a technicallity, it'll prolly do 100% better.
The Earth has been around for a long, long time. You can look at how Chernobyl has been restored, to an extent. Plants are growing and animals are living there once again. As far as humans moving back there, I don't think they will any time soon and not being a scientist, I don't know the lingering radiation in the area.
Now, this left to itself, we "have reason to believe the world will end 12-21-2012" based on the fact that they Mayan calender ends on that date. My personal guess is quite simple. These people just disappeared off the face of this planet and at one point their land was invaded during the Spanish Inquisitions, which is part of the reason, in my opinion, these people seemed to fall off the face of the Earth. They were 'assimilated' for lack of a better word, into a combined people with the Spaniards. I'm saying it's safe to assume they just so happened to stop making their calenders for whatever reason and the last day they got to was 12-21-2012.
Not contesting that the Mayans had technology or intelligence beyond their time, just giving my .02¢
Besides, I've grown quite bored with all this talk of "The world is going to end on x-y-z." It's getting old.