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Yep Ken, the Earth's magnetic fields do shift.....the period of the shifts isn't a set number of years, but there appears to be, from the rock records, a series of shifts which occur....some on a 50000-75000 year timescale, others on timescales up to a million years in length (but usually around 675000-750000 years). Our atmosphere is more than thick enough to protect the life on the surface of the planet, but I wouldn't be too happy for electronics and the electricity grid. Navigation would be a problem especially when you can have up to a dozen or more weak magnetic poles present when the field is down to the minimum. Travel to space, unless you had good shielding, would be hazardous and satellites would be in trouble. Could even be some climate changes occurring (increase in aerosols and such due to more radiation being allowed to enter the upper atmosphere, changes in condensation nuclei size, shape, composition and longevity etc etc etc) because of the flip, but nothing that would mean imminent disaster.
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