Some people have _claimed_ that the planets will align in 2012. Celestial mechanics is one of the most exact sciences, and we know exactly how the planets are going to move for thousands of years into the future. In fact there are many computer programs which show this. When I heard that there was going to be a planetary alignment in 2012, I naturally checked it out with Starry Night software. I found not a single planetary alignment in the whole year, not even of two or three planets, let alone all eight. In fact, there hasn't been a complete alignment of all 8 planets in the entire 5 billion years the Solar System has existed.
What is going to happen on December 21 2012 is that there will be a rough alignment of the Earth, the Sun, and the centre of the Milky Way Galaxy. When I say rough, there's a 6 degree bend in the middle of the line. This is no big deal, because this same alignment happens EVERY year on the same date, and has for hundreds of years. It happened a few weeks ago, in fact, on December 21 2011. Did you notice anything?
When this "alignment" happens, what does it mean? Absolutely nothing. Gravity is a force which decreases with distance. That's why the Moon has a major effect on the Earth's tides while the Sun, which is much more massive but farther away, has very little. The centre of the Galaxy is of course much more massive than the Sun, but it is so very far away that it has absolutely no effect on the Earth. To get some idea of the relative sizes of the distances of Sun and Galactic Centre, the Sun is eight light MINUTES away while the Galactic Centre is 27,000 light YEARS away. Figure out how many minutes there are in a year, and divide by eight, and then multiply by 27000, and that will tell you how many times farther away the Galactic Centre is than the Sun (about 2 billion times farther!)