What will happen when the Earth's magnetic field reverses itself? We may find out in just a few short thousands (or tens of thousands) of years!
The Earth's magnetic field has reversed repeatedly over the course of geological history. During such an event, the magnetic poles flip, presumably affecting the magnetosphere (which helps to guide solar radiation to the poles, where the Aurora Borealis and Aurora Australis occur). The most recent reversal was ~780,000 years ago, and we don't know much about what happened then. It seems that we could be in for another reversal "soon" - although when is a matter of some debate.
When the next reversal occurs, interesting things might occur: compasses will be affected (which would also have a big impact on GPS and mapping in general), migratory birds and fish may become confused, and some geologists think that the reversal could cause volcanic eruptions (due to stresses in rock strata). But of course, we don't know for sure, since we haven't been around to witness past reversals.
There's plenty of interesting material on the web about geomagnetic reversals: a May 10 article from the Economist, a surprisingly thorough Google Answers post, and Wikipedia on geomagnetic reversals are good places to start.