There's evidence it happens to the Earth too: http://en.wikipedia.org/wiki/Geomagnetic_reversal http://en.wikipedia.org/wiki/Geomagnetic_reversal seems like it'

There's evidence it happens to the Earth too: http://en.wikipedia.org/wiki/Geomagnetic_reversal http://en.wikipedia.org/wiki/Geomagnetic_reversal seems like it's probably a fairly common thing with any spinning core.