Ok, lets say there WILL be an astronomical event that will affect the denizens of earth, and the government has known this for decades and have prepared. How will that change us? We will still be affected by the same event, we will still have to continue our daily lives and eat. And if the government has created so called "Shelters," how are we suppose to break into a military fortified bunker designed for high valued personell?
The point is, even if someone confirms with hard proof that the government has kept a secret about 2012, nothing can be changed. In fact, not telling is the better option, since broadcasting the news of an upcoming disaster will cause worldwide panic and a breakdown of government and society.