The Earth's geomagnetic field is well known to anyone who's ever used a compass. Such usage is typically on the surface of the earth, but it must be noted that the geomagnetic field permeates the planet itself and extends outward into space. At the surface of the Earth the field is evidenced by the behavior of magnetite material, such as bar magnets, which when suspended and free to rotate, align themselves with the geomagnetic field.

According to paleomagnetists, the Earth periodically -- every few hundred thousand years -- flips its magnetic field (i.e. the north and south magnetic poles are reversed). Evidence for this peculiar habit is provided by measurements of the orientation of magnetic materials in sedimentary and volcanic rocks, and the fact that such orientations have been grouped into distinct periods over the last 178 million years. Based on this evidence, the most recent magnetic pole reversal appears to have occurred some 730,000 years ago, leaving the north and south magnetic poles at 75 degrees North, 102 degrees West, and 67 degrees South and 140 degrees East, respectively. Of course, this means that based on the relatively recent past, we're way overdue :)
Check out these sites for more detailed info:

Pole shift hypothesis -
http://en.wikipedia.org/wiki/Earth_Crustal_Displacement

Magnetic Pole Shift May Be Underway -
http://www.rense.com/general51/mag.htm

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