My understanding is that the Mayan's predicted the end of the era in 2012. The era began roughly 12,000 years ago (interestingly when the last ice age ended), and encompasses the whole of human 'civilisation'.

In order to accept this belief, you also have to take on board the belief that there were a number of previous eras, containing various levels of human civilisation. (Score one for the Atlantis theories - the Mayan calendar describes the end of the last era in flood, at just about the time that Plato put the flooding of Atlantis. There is some faint evidence for flooded civilisations from this period off the coast of India)

My personal approach is that if 2012 marks the end of human civilisation, it will be a huge bonus for the UK who will be spared paying for the olympics (and all that national debt). I plan on buying my Christmas prezzies very late. Other than that there's not much point in getting worked up: if it's going to happen (which I rather doubt!) there's nothing will stop it.

I have seen a theory that put 21st December 2012 as the date we finally get the LHC functioning...