I'm partial to the "time to make a new calendar" theory.
From what I've gathered, the Maya were accomplished enough astronomers that they could observe, calculate and therefore predict the cycles of the spheres accurately.
I assume the cycles don't repeat in some endless looping clockwork, so there are only so many celestial wanderings that can be recorded on one stele of moon phases and googly monsters.
Therefore, time to make a new calendar where the old one left off.

As for the "Dark Planet" theory, you'd think the Maya would have written down such an important detail
somewhere as the reason for ending the calendar when they did, no?
So far as I'm able to discover, the calendar simply ends; no associated cataclysm or doomsday prophecy (beyond the ENDLESS speculation being done by the New Age crowd).
In fact, in all the research I've done, I can find no associated archaeological record or discovery in any meso-american culture in which they recorded that not only does the calendar end, but it will end with a bang.
I believe the only reason this subject is even being discussed is that:
(1) It is a concrete date.
No other civilization lost to the sands of time has given so unambiguous a date for any event, no matter how mundane.
(2) The date in question is "at our doorstep", so to speak.
Whether we take any stock in it or not, it is something happening
in our very lifetime, so it enters into our culture and our collective conversation whether we like it our not simply by virtue of the fact that it is NOW.
Personally, I like the Bible's proclamation that "no man will know the day or the hour", so basically setting a date is kinda pointless.
P.S. Here's an easy read on Mayan math and date reckoning:
http://www.buriedmir...om/maya-calendar.htm