The news media has publicized that 2012 is the end of the Mayan calendar, and that all the planets will be aligned. Is there any truth to this?
The answer is absolutely not. There is nothing interesting/important/rare/weird/abnormal/whatever astronomically happening in December 2012. I have written at length on this topic and a year ago summarized my posts on the subject. In those, I've covered the vast majority of 2012 doomsday or whatever claims. Besides that, though, 2012 is likely not the start of the next baktun in the Mayan long count calendar.
With that stated, one astronomical event that is rare that is occurring in June of 2012 is a Venus transit of the sun. This happens in pairs of 8 years separation with gaps of 121.5 and 105.5 years between those pairs. The last transit was in 2004, the next in 2012, the one after that in 2117.
I have thought for many years that the current transit of Venus, mentioned above, is the reason why the Mayan calendar "ends" this year. On the multi-decade or multi-century scale, this time is singled out by the equinoctial precession having placed the solstice sun in proximity to the center of the Milky Way bulge, and the precise year is singled out by the second transit of Venus. This refines John Major Jenkins' idea that this is the year in which the solstice sun comes closest to the "galactic center", as seen from earth. That's not true and the exact center doesn't stand out to the naked eye anyway, but the general precessional trend and the timing of the transits were both accessible to Mayan observers.