Although there are many reasons that the world may end in 2012.
The true reason will be caused by an impact as we move into our 28,000 year cycle through the galaxy.
The Planets in our solar system rotates around our Sun, and our Sun rotates around the center of the Milky Way Galaxy. But our galaxy does not move on one plain, but rather in a circular motion that travels from high to low (rather like a hoola hoop spinning around your waist).
Every 28,000 years our solar system reaches a "plain" in our galaxy and is aligned with the center of our galaxy.
It is on this "plain" that most of the debris left over from the creation of our galaxy exist (Asteroids, Comets, etc). Scientists have discovered that we will enter this "plain" in 2012. And at that time, I believe our planet will be pummeled by these debris.
Maybe even by another solar system or Sun crossing our path?
Look at all the near earth objects that we have been encountering. The last one missed us by only 1.5 x the distance from the earth to the moon (That is really close in cosmic terms).
and it's only going to get worse. And with the changing of the earths magnetic fields in 2012 (After completion of the earths wobble) we will not have any protection against even the smallest atmospheric intruders. Look at the small asteroid that crashed near Yakima, Washington.
I think we all better get out our telescopes and try to locate what's coming so we can all head to high ground and play dodge ball?
More info can be found here
http://alignment2012.com/whatisGA.htm