The world will not end on December 21, 2012!
Although it's going to get a lot nuttier than the Millennium change before we get there.
December 21, 2012 will be the Winter Solstice, at midday, the sun will start moving back north. It does that every year, on about the same date.
The difference in 2012 will be that the sun will rise in the constellation Aquarius for the first time in about 26,000 years. Before American Idol, what people did at night was look at the sky. They though all them things up there had effects down here. They also believed that throwing virgins into volcanoes was a good thing so they did get lots of things really wrong.
We are now in the Age of Pisces -- and the dawning of the Age of Aquarius. On the 21st of December 2012, the Age of Aquarius will finally be here, at dawn (dawn being relative to where you are on the planet). In another 26,000 years or so we'll all be back at yet another dawning of the Age of Aquarius. The rotation through the entire zodiac is called the Long Year and is the result of the Procession of the Equinoxes.
It's an easy prediction, whether by Mayans, Nostradamus, or your crazy uncle Fred. Mostly, it's the History Channel going "boo!"