Nibiru does not exist, and a non-existent object cannot hit the Earth in 2012. It is perhaps believable because it has had so much time to sink in, so much media attention, and so much authority (albeit it false) plumping it up. For a start, no Sumerian texts even suggest the existence of such a planet (ancient texts scholar Michael Heiser has covered this on his website).
Also, professional astronomers have no data to suggest the existence of such a planet. The Planet X Arguments (Bad Astronomy website), author Phil Plait points out that the planet was not detected by NASA in the early eighties, despite popular belief that it was (and that it’s still being secretly monitored). Some objects were found in the early eighties, but the possibility of one being a planet was quickly refuted.
However, space agencies aside, there is no cosmic umbrella that stops amateur astronomers from investigating. In his article The Myth of Nibiru and the End of the World in 2012 (Committee for Skeptical Inquiry Website), author and astronomer David Morrison describes astronomers (both professional and amateur) as “a free-spirited group who couldn’t keep a secret if they wanted to”.
Is it worth preparing for, since we can’t really know? On the face of it, this stance sounds sensible. However, there is much we “can’t really know” that we don’t prepare for. We can’t know with a 100% certainty that a comet won’t hit us tomorrow, or the next day, or the day after. The difference between Nibiru and my comets is that my comets haven’t been dressed in the same hype, sensationalism and outright lies.
Just look at similar doomsday events that turned out to be untrue. Nothing bad happened when the planets aligned in 2000, and no black hole gobbled the Earth when the Large Hadron Collider was switched on in 2008. Perhaps more importantly, the very same Nibiru did not flip the Earth over in 2003 (my source here is that I think I would have noticed).
I’ve only been able to look at the tip of the iceberg here, and fortunately there are some sites that go into depth. An excellent recent article on this topic is The Myth of Nibiru and the End of the World in 2012, published on the Committee for Skeptical Inquiry website. A great site that covers the myth in detail is Planet X and the Pole Shift.