JOEL - I know that there are many such calendars and traditions that indicate a significant event happening during the winter solstice in 2012, including some Native American traditions.
Many believe that, according to the Myan calendar, it represents the date that the world will end. That is however unlikely because the Myan calendar continues on again for another cycle after that date for thousands of more years. It may only mark the end of one era and the beginning of another, sort of like the odometer of your car turning to 100,000 miles.
The records speaking of the significance of this date only say that time will change or cease to exist as we know it in some way (whatever that means). There will also be some interesting astronomical alignments that will happen at that time, but they are not expected to have any impact on the earth.

Personally I do not believe that any calendar or Myan tradition can predict what is going to happen to mankind on a specific date. I do not think the world will come to an end at that time.
"Watch therefore, for ye know neither the day nor the hour wherein the Son of man cometh." (Matt 25:13)
The only significance this date has for me is that my granddaughter will celebrate her ninth birthday on Dec. 21, 2012 :-)