I think that Planet X is a myth, and the Mayans did not continue the Mayan calendar past 2012 because there were no more Mayans left to keep it up. The Mayans were wiped out. What makes them so wise anyway? I am actually shocked they made it that far. :)
Unfortunately what believers consider to be the "truth" behind the allegations that we will be crossed by a certain Planet X (otherwise identified as Hercobulus (sic), Nibiru, et al.) is what we, the skeptics, file under "speculation" and "sensationalism."
What your question is refering to is a current meme which states that, in the year 2012, our big blue marble, Planet Earth, will suffer a catastrophic crash or near-crash with a planet that has been out of orbit since Jeeves knows when. Furthermore, people who believe in this idea feel that they have the back-up of Mayan ancient knowledge to prove that this will, in fact, occur.
That cannot be categorized as a "truth." The reason is simple: A true statement has to be evidenced by factual documentation that, as of this day, cannot be produced when it comes to when exactly these events will take place, or when will the world "end."
However, there is one very true thing: It is true that the Earth has been "doomed to extinction" since the beginnings of history. The Bible calls it the Apocalypse. Other books call it other things. Why? Because everything that lives, at some point, must die. The intrigue of wondering how such a lonely planet will expire is too tempting to resist speculation.
Just to add my two cents, the world has been "ending" since, basically, forever. It has had more expiration dates than anything I've ever seen. The world had been supposed to end in the years 666, 1666, 1984, Y2K, and now it is supposed to be in 2012.
Oh! Don't forget how we did not end as a world this year either, and that doomsday was conveniently moved to October of 2011.
In conclusion, to avoid further sarcasm, there is no "truth" to it. There is only speculation, fed by sensationalism and completely pumped by utter ignorance.
The Planet X information comes from the interpretation of Sumerian tablets by Zacharia Sitchen. I don't know if there others who had the same interpretation, but I don't believe there are too many that tried.
I believe like the previous poster, that the Mayan 2012 date makes for an opportune time to link the 2 events, but there is nothing that binds them other than speculation.