Well, either it will be nothing, and I hope it's not a pole switch as then all electronics will be screwed (at least I think) but it will likely be nothing.
However, if it is something here is what I believe will happen. The Bible mentions a race of people called the Nephillim, who were giants.
Quote:
1. When men began to increase in number on the earth and daughters were born to them,
2. The sons of God saw that the daughters of men were beautiful, and they married any of them they chose.
3. Then the LORD said, "My Spirit will not contend with man forever, for he is mortal; his days will be a hundred and twenty years."
4. The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown.
Basically, the Nephillim are the children of fallen angels. I believe these fallen angels in the Bible can now be associated with the "aliens" people see today. So, IF things happen in 2012, and something bad happens, I think what will occur is, first contact with "aliens" however these space aliens will be demonic in nature. These "aliens" will then say they're our true creators, not God, and then the "aliens" will experiment and do all kinds of bad s**t to humanity.
That's the worstest most horrible case scenario, and I'm not sure God will allow such a thing to occur, but if he did, then that's what I see as the "plan" in the Mayan calender.
But likely nothing will happen.