Has anyone ever thought that the Mayan calender maker may have run out of room on his rock?
either that, or maybe the witch doctor who made it said to himself "leave it like this, man this is gonna mess with someone someday" 
LOL, good one.
Actually, the Mayan calendar is a cycle of some number (10, 12??) of smaller calendars. When it hits the end (as it has once or twice before), it just restarts at "stone #1'.
I think we have a LOT more to fear from earthly forces in 2012 than by anything cosmic or geologic (do nukes count as geologic, though??).
No matter what, the earth isn't going to simply stop turning. Lighten up, it'll be ok.