What's going to happen in 2012?

First off, the world isn’t going to end in 2012.
It’s already over. I’d say it died in the late 80’s or early 90’s.
What we’re living in now is the birth pangs, the labor, of our new world being born.
The “end date” of 12-21-2012 (the Winter Solstice) comes from the Mayan Long Count calendar. Without getting into huge detail on how the calendar works (that will be another article), the Long Count date on 12-21-2012 is 13.00.00.00.00. Definitely a number to make you say “hmm—What’s up with that?”
And indeed, something is happening on that date, no matter whose calendar you use to track it.
Due to the precession of the equinoxes (yet another article), every day the sun comes up in a slightly different place. We don’t see this because of how slow it moves (it’s a 26,000 year cycle), but it’s happening. On the morning of the Winter Solstice of 2012, the Milky Way will be at the horizon. And the sun as it rises will go through the dark area.

Photo source: Nasa Astronomy Picture of the Day archives.

This dark area was thought to be the birth canal of the universe by the ancient Maya. What happens when something goes through the birth canal? It’s born.
Or re-born, as the case may be.
What the supposed “end date” is actually pointing to is a new beginning, a new world, a new sun. The Aztec Calendar stone depicts a central figure (Tonatiuh, the sun god) with four smaller glyphs around him. Those four glyphs are the previous world ages, or suns. Tonatiuh represents the fifth sun, the sun of movement—the current sun which is dying/dead.
The Maya looked ahead and saw the rebirth of the sun, the sixth sun, on 12-21-2012. It’s a birthday celebration, not a lamentation.
Written: 12.19.13.3.11 12 Chuen 4 Pop (manifestation portal)
2449 days until 12-21-2012
April 08, 2006

A good source for technical info on the calendars:
http://www.public.iastate.edu/~rjsalvad/scmfaq/calendar.html