12/21/12 is just the end of the Mayan long calendar, which doesn't signify anything at all except that you start the calender over.
As for some of the other stuff that's tossed around for this date, magnetic poles switching, crossing the galactic plane, sun activity... please!! These are all things which have already happened repeatedly through human history. Sun activity being 11 years, and the others in tens of thousands of years, but still, humans are estimated to be 1m years old on the planet, so obviously, none of this will matter in the big scheme of things or we'd have died out long ago.
I haven't seen one convincing bit of evidence that that date means anything at all, except I now only have 4 more days to shop before Christmas, and that will probably be the biggest calamity.