Update:
Nibiru continues to have been a name used by the Sumerian civilization for the planet we call Jupiter.
Nibiru is still the name of a fictitious planet used in Zechariah's book (written in the 1960s) where Z invents a story about a people living on a planet that has a strange orbit bringing it close to Earth every 3700 years. In his original story, the planet he calls Nibiru (not the same as the one called Nibiru by the Sumerian) cannot harm Earth when it comes back. A few years ago, Z published a letter where he confirms that HIS planet Nibiru cannot return before the year 2085.
Z's story is "based" on Sumerian myths the same way that Stargate (the original TV series) is based on Egyptian myths: it is just a ploy to pick names of heroes and villains, and a convenient way to make up an old common language used on the different planets of the series (otherwise, it might be difficult to explain why the Earth scientist can decipher the ancient scripts so easily). Similarly, Z's story is loosely based on some information that he found in ancient documents and on twisting (a lot) some of his own interpretations of texts in the Bible.
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Update on a totally separate hoax that was not connected at all to the Sumerians when it was invented:
Planet X was the planet that was to wipe us out in May-June 2003. It was invented from Nancy's dreams (she claimed that her dreams were secret messages from aliens). Charlatans used Nancy's dreams to construct a hoax and make money from it: they sold books on how to survive and some even sold seats on alien spacecrafts sent to save a few of us (back in Spring 2003). [The spacecrafts became the Arks in Roland's movie "2012" which, despite its title, is really based in part on the 2003 hoax).
After June 2003, the charlatans needed a new hoax, so they created the Big 212 Hoax. They used their own Planet X hoax. They used other hoaxes, lies and stories, modifying them to fit their own hoax.
That is how, for example, Nibiru was stolen from Z and forced to participate in the Big 2012 Hoax (for that, the charlatans had to modify its orbital period, otherwise it could not come back in time for 2012 -- that is why some people use 3600 years instead of the original 3700). It is in response to this obvious copyright infringement that Z wrote the letter confirming the year 2085 as the earliest possible return of Nibiru.
The date of the Big 2012 Hoax comes from yet another lie, invented by a different charlatan (claiming to be a reincarnated Mayan priest) in the late 1980s: the end of an old calendar used in Central America. The calendar is really the Mesoamerican Long Count calendar. It does not end, but it merely passes a round figure, as it does every 394 years and a quarter.
Update on the update:
When Planet X failed to show up in 2003, Nancy said that aliens apologized for making a mistake about the date, and they announced a new date in 2011 (not 2012). But the charlatans had already made up the Big 2012 Hoax, so they did not want her to be too "public" with her new date. They figured that their hoax date was far more popular than hers.
In any event, her date would conflict with two end-of-the-world dates already announced for 2011 by some obscure Bible-cults in the USA (check out May 21, 2011).
Next update will be published on January 2, 2013.
By that time, the "new" date for Planet X will have been announced by the charlatans who will have taken over from the present charlatans.
Personally, I am rooting for September 20, 2020. In addition to its numerological appeal, it also happens to be an important date in yet another ancient calendar.
After that, you can pick your own "end-of-the-word" dates (yes, with an "s" -- you should pick more than one to increase your chances in the pool).