The earth's magnetic field has reversed itself MANY times in the Eath's 4.6 GA history. But, since scientific humans haven't been around to document the actual reversal itself, the process by which the field actually reverses is somewhat speculation. Like another poster has said, based on previous periods of magnetic field reversals, we are long overdue for one.
As far as the Earth's core, from seismic data and density/gravity etc modelling, the inner core is solid and made up of Fe, Ni and other metals. The OUTER core is molten and that is what is circulating and probably creating the Earth's magnetic field.
Most planetary geologists believe that the reasons planets like Mars or even our own Moon are solid with no convecting core is due to their size. Earth is bigger and thus takes longer to cool.
Finally that Geology degree has done something useful!
Now everyone get your tin foil hats!