Geez guys. How about a little research before you have the normally well-informed denizens of Orbiter confused with the rest of the Great Unwashed.
Nibiru, the supposed Planet X, is mainly from Zachariah Stichins questionable interpretations of Babylonian/Chaldean/Mesopotamian astronomical scripts already describing the bodies of the solar system out to and including Uranus and Neptune. Nibiru, not necessarily being in a highly elliptical orbit but a very high inclination orbit, is supposed to pass between the orbits of Mars and Jupiter. It has been searched for for a long time ... no success. Perturbations in the orbit of Neptune and Uranus were accounted for when we finally determined an accurate mass for each ... by passing satellites close by.
Not that Stichin was necessarily wrong. Other reputable translations do indicate that Neptune and Uranus were known to the Mesopotamians, thousands of years before the 20th century CE. Food for thought.
Nemesis, is the supposed "Dark Sun", sister star to our sun. There are no gravitational anomalies to suggest its existence. Even a brown dwarf sister to our sun would have been discovered by now.
Want to make a suggestion about a possible dark matter object? Just might bear out the predictions in the Man-Kzin Wars series. No worse fiction than anything else put forward to date.
As for 05 May 2012? Again based on questionable interpretations of the derived end of the current epoch of the Mayan calendar. Since we still have not worked out all of the symbols in the Mayan glyphs yet, what is the authority?
"Predictions" of apocalypse almost always seems based on someone's attempt to extort valuables from the ignorant masses, or attempts at control by fanatical religionists. There have been many. Many seem to end up being the reasons for new sects to be formed (Mormonism and Seventh Day Adventists come to mind ... don't flame or cry to a moderator, look it up). In the end, in the paraphrased words of Heinlein, it is no sillier theology than any other.
Best way to discuss it? Over copious amounts of flavoured ethanol or your favourite mixture of blue smoke. Always entertaining.