I was under the impression that it was the Inner Core getting started again, my apologies.
The bombs don't generate a current -- they re-establish motion in a non-linear fluid dynamic system of the outer core. It's a little fast-and-loose, but a hell of a lot closer than the "split the asteroid the size of texas with a single nuke" gig. Once again. Sci fi. You don't choke on faster-than-light travel in space movies, you won't choke on this. Of course, if you do, then you're an enemy of fun.
Wouldn't heat differences form between the poles and equator, and begin the process again with more energy than a measely thousand megatons ever could?
Sorry, had to pick
The Armageddon quip is true, and suspense of disbelief is always necessary, but understand it seems to be easier if you go to a different world where these changes in rules are par for the course.
The outer core also, as xeriar points out, does change rotational axes every half a billion years or so. That's why our north and south poles flip. I'm a little confused over your assertion that that movement isn't primarily responsible for the geomagnetic field, but I guess research sources can vary. The JPL guys seemed okay with it.
More like half-million

We're on an unusually long stint right now at seven hundred thousand years. Also, though the overall field collapses, the field strength actually stays the same - it's just confined to countless local eddies that conflict with eachother.
This process is occuring now, which is where we get the 'half life of the Earth's magnetic field' from the Creationists :-/
As I said, I was under the impression it was the inner core since stopping the outer core would just be... weird. Way weird, I would expect volcanic more than storm activity as temperature differences built up, although I suppose, that would be over geological time, and that just kind of ruins the suspense in a movie
No extinctions can be tracked to the reversal, but that's because it's always gone smoothly up to this point. As for the effects from the collapse of the electromagnetic field, google up "Dr. Marvin Herndon". He recently published a peer reviewed paper for the Academy of American Scientists which proposes a potential collapse of the geomagnetic field (from a different origin than ours, but the same end) and confirms that the ensuing disasters would not only be similar to the ones in the flick, they would be worse. Nice bit of synchronicity actually.
I have two problems with people claiming this, though
1: The Inuit and residents of Antartica live under similar conditions, as far as solar storms are concerned. If they could survive, for so long, why couldn't the rest of us?
2: The reversal has happened several times within the period of human evolution, so 'everyone on the planet is dead' seems a little extreme to me.
It is true, however, that the Toba Supervolcano (if that is really what caused the 'bottleneck', of course, but I'm inclined to agree) would have eliminated any genetic trace such a weather event would leave. Not to mention Yellowstone erupted before the last shift as well.
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Regardless, you've given me a bit more confidence in the movie - I may get the DVD - I don't go to the theatres much

Congratulations and thank you.
As for killing Orcs in my basement, a part of that is actually tied to this - sometimes I just wanna toss all this real-world crap out the window and play God with my own rules, and see how I do. For some reason I do better with that as a premise
And for faster-than-light travel, at our current rate we will exceed the speed of light Circa ~2200. Now, if we can beat time dilation and REALLY do it, that would be something.
(You can go as fast as you want, a trillion times the speed of light, or whatever, but the rest of the universe still sees you plodding along at .9999999... of c)