There are two types of pole shift, and these types are quite different, so this site has two halves. As an introduction, here is a scientific overview of each:
Orthodox science accepts that this is a real phenomenon. Which means it will happen again, one day.
The Earth's rotation and orbit are unaffected. The Earth's crust stays where it is. The change is in the Earth's Magnetic Field. Like a giant magnet, our planet has a North and a South, and we can locate the magnetic poles with a simple compass. The most accepted model for our magnetic field is a dynamo action resulting from the movement of molten iron in our planet's core.
Our Sun has a magnetic field, and this field reverses every solar maximum, which are approximately 11 years apart, and the next one is due in 2012/2013. While the Sun's reversal happens at the peak of magnetic strength, here on Earth it probably happens when the field reaches a low point. The last full reversal (that we know of), was 780,000 years ago, and there have been a total of 184 polarity intervals (that we know of) in the last 83 million years. The timing for each appears to be random.
This concept is not accepted by orthodox science, although it has had some distinguished fans, including Albert Einstein. While there is evidence for displacements in relatively recent geological times, the sticking point is a lack of internal mechanism.
With a displacement, the Earth's outer crust moves rapidly (as opposed to the extremely slow movement due to plate tectonics) and slips relative to the core. The result is that land masses end up being located at a different latitude/longitude than before the shift. A famous piece of evidence for such a shift is the graveyard of mammoths in Siberia. The mammoths were snap frozen. One specimen was chewing on a plant when he died - a plant that does not live in arctic zones. The supposition is that the mammoths were living in a more temperate region, and a sudden pole shift transported them to the Arctic.
According to Charles Hapgood, the shifts were roughly 30-40 degrees in nature, and the last one occurred approximately 12,000 years ago.
A crustal displacement is featured in the recent blockbuster movie 2012.
There is actually a third type of pole shift, where planet Earth's rotation reverses. This would mean that the directions of sunset and sunrise would reverse, East would become West and West would become East, as described in the Bible and other ancient texts. This is not discussed much in the modern era, primarily because a reversal of rotation is not possible without an outside agent that would either destroy the planet, or remove us from our pleasant orbit around the Sun.