This has been something that fascinated me the first time I heard of it, the North and South poles of the planet shifting. Recent work on sediments in a lake have given a much better estimate of the time it took and reduced it from "less than 1000 years" to "less than 100". Further, the lack of sediment showing inter-pole positions means it might have been practically instantaneous. That would be a pretty wild thing to live through.
The standard answer is that the magnetic field protects us from part of the radiation that comes from the space. I tried to find a good source with more information. I'm still not sure, perhaps it's only a myth.
This page says that if there were no magnetic field, the atmosphere would absorb the radiation anyway. (I vote for this, but I'd like to see more info.) http://www.sciencebits.com/NotReallyAProtection
> During this change the strength of the magnetic field dropped to 5% of its present strength.
> The magnetic field will not vanish completely, but many poles might form chaotically in different places during reversal, until it stabilizes again.[
Hard to say, the Northern lights would move to the other hemisphere, the entire sky might glow if the field were not present (solar ejecta hitting the atmosphere everywhere)
It is possible that the Van Allen radiation belts would vanish and reform in a different configuration. That process might damage satellites.
A big chunk of ozone would get 'zapped'. The reason there are holes in the ozone layer at the poles (human cfc's not withstanding) is that highly charged particles can break it down[1].
