alright, so as you probably know, many of Nostradamus's' quatrains that have been 'true' are really just a couple lines together that somewhat, maybe, vaguely resemble some incident and people relate the event & relate it to the quatrain, so that it somehow seems like a prediction.
yeah, its not very prophetic work if you ask me because his words mean nothing unless an event is molded to fit it. and I'm fairly positive he never ever wrote dates, which would put me in doubt of the "Nostradamus predicted a comet (Mabus or something) will crash into us in 2012" part of your question.
BUT.
the Mayans predicted that in 2012 the fifth world (which is our world) will end. [the first four worlds were believed to be ended by comets, floods, volcanoes, and something else.]
The Mayans also predicted that a white male with a beard riding upon a majestic four legged animal would appear the exact day that Hernan Cortes, who fit that exact description, arrived. So if they can predict their own demise, among many other events, I respect their predictions.
I also believe that the world will line up with the other planets on like December 21 of 2012. Maybe this is what both the other were alluding too? Maybe they believed that if the planets aligned, the world would end. We will never know, but might as well have the time of your life on December 20th 2012 just in case.