I've been hearing about this since the late 90's, and one of the dates considered for a pole shift was on May 5, 2000 when there was a planetary alignment. I remember there were books about it in new age stores talking about how the world would end, or much of it destroyed, etc.
I think there was a grand cross in August of that year or 99 as well which new age books were saying was a sign. From what I remember the planetary alignment was in Taurus so it was symbolic of earth changes and it was thought there would be such a great focus of energy with the planets aligned that it would wreak havoc on the earth. As I recall, the story was the earth was going to shift on it's axis, so the north and south poles would not necessarily switch, shifting about 90 degrees so the equator's would be where the poles are now. Nothing happened on a grand scale that was evident though and after that I think the story moved towards the pole shift actually being a more gradual shift rather than a sudden cataclysmic shift, and earth changes would follow such as an increase in earthquakes, volcanoes, weather changes, etc.
Now I guess maybe the idea moved into 2012, which has long been forecasted to be a time of great change. Some think it will be physical, like an Armageddon, others think it will be a shift in consciousness, and some think it will be a shift into another dimension which I think has a lot of differing idea's surrounding it.