I actually did some research on this a while back. A really interesting movie to watch if you're interested is Zeitgeist (Google it).
So, the theories:
1. Christian ideology: December 21st 2012: Astrologically, this date marks the end of the 'age'. The earth is on a very gradual wobble, and every 2150 years, the sun will rise in the area of a new astrological constelation. Currently, we're in the age of Pisces - the fish. This is the same age of Jesus's birth, but the Jewish (Old Testament) Prophet Moses was born in the age of Taurus - the bull. The ancients were well aware of these ages, and they correspond with religious symbolism and the Mayan calendar. Some Christian thought-groups (extremists) believe that the world will end December 21st 2012 because in the Bible it says that Jesus will return at the "end of the Age". In some translations it appears as Age, some as Era, some as Time. For those who follow the 'Age' interpretation - December 21st 2012 is when that Age ends and judgement day will come.
2. The Mayan Calendar. The Mayan Calendar is an astrological calendar developed by the ancient Mayan civilisation. What's freaky about this ancient calendar is that it ends on Dec 21 2012. The theory of the physic/New Age world is that the Mayan calendar represents human development in the metaphysical sense, in terms of states of consciousness. It goes in 'days' and 'nights of certain lengths, in a pyramid shape, each equal in length although getting shorter and shorter as we get closer to 2012. There are then levels composed of theses days and nights, each level represents a new state of human consciousness. For example, action / reaction is one of the earlier ones. Historical Mayan sources suggest that the end of the calendar is cause for celebration, which is agreeable to the New Age beliefs. Dec 21st 2012 will bring a new heightened state of consciousness, and people in tune with their 'aura' will feel a significant energy shift in the world.
They're the two main theories I know about. It's such an interesting topic. There are others, of more apocalyptic tendencies, and the date coincides with a few significant astronomical events. Some people think it's to do with the earth's polarisation, which is reflected in the increasing frequency of global natural disasters.