Pole Shift: A magnetic pole shift wouldn't destroy earth, but i would put us back in the stone age. The effect would be similar to the EMP burst of a nuclear weapon... Only much stronger. Changing magnetic fields produce electrical current, such a massive field reversal would fry EVERYTHING, even the things we considered "shielded" from such an event.
You can debate the severity of the aftermath of this, but the fact remains that populations would be isolated. No cars, no power boats. Cities would run out of food first, hysteria, etc....
Also, such a massive burst of electromagnetic activity would almost surely cause problems for nuclear powerplants, deactivating cooling systems and the like... VERY BAD!! Missle silos would lose control of the missles, they would not be able to fire, but the arming mechanisms would be fried... potentially very bad.
Actually, considering that the Earth's magneitc field is only .6 gauss at its strongest point, the EMP effect would be quite limited. You may loose a few of the less-protected power girds for a while, but cars and other devices wouldn't even be effected in a real EMP burst. Plus, without the electron shower, computers and cell phones would DEFINATELY not be effected.
It is not the strength of the magnetic field that would be the problem, it's the size. Magnets and Elcetricity are very closely related (hence the term "electromagnetic"). An abrupt change in the magnetic field of an entire planet would result in catastrophic damge to electrical infrastructure, the amount of energy needed to shift the poles would be shed in the form of radiation. Ever wonder why every electronic device sold in the US has to have the "complies federal law section XXX this device shall not cause harmful interference..."? Those types of devices are very suceptible to even weak bursts of this kind. Put a magnet next to your cell phone and see what happens. Across the entire planet, communication is gone. No food. No idea what happened. What would you do?
About the tectonic implications of this, it is more dependent on the speed at which the pole shifts. If the core violently lurches, everyone would feel it... Tsunamis, earthquakes, eruptions... it would not be a good time.
Not to mention that the magnetic field is one of the main reasons why we havent been killed by radiation yet, what would happen with a random rapid pole shift and the radiation shield, nobody knows.
About a black hole swallowing earth, very little chance this could happen. There simply isn't anything with enough mass in our solar system that would create a black hole if it collapsed. But, for all we know, the sun is heading towards a black hole right now and taking us along with it.
The "end of the world" and the end of civilization are different things; which one, if either, is going to happen in 2012? Who Knows.