The two standing theories on the earth’s magnetic field are 1) it will collapse and reform, 2) the field rotates and magnetic north will move.
What effects will this have? Please be brief, clear and concise. 
Nothing.
Way back in my pilotage days, I’m fairly certain that I learned that nautical charts all contain a way of calculating the actual position of magnetic north. The compass rose has the number of degrees from true north, a date and a number of degrees per year that magnetic north drifted. (Obviously, every chart will be slightly different, since the angle between true north and magnetic north will depend on your position on the globe. In addition to variation [the difference between true and magnetic north], we had to include deviation, which is caused by the magnetic field of the ship itself.)
So, I’d guess that magnetic north moving is a pretty well accepted fact, known for quite some time.
There is another possibility. The magnetic field consists of a strong dipole field with weaker higher order fields such as quadrapole, sextapole, etc.
Prof. Stephen G. Brush of the University of Maryland[sup]1[/sup] cites McDonald and Gunst (*ESSA Tech Report IER-46-IES 1, * Government Printing Office, Washington, D.C.) who made a study of the history of the magnetic field from 1835 to 1965. They state:
In other words, the total magnetic energy is pretty constant but is being distributed differently among the dipole and other modes. So it’s entirely possible that the field will stay about the same overall strength but will not have definite N and S poles like now, and then reorient itself again into a main dipole field probably reversed from its present orientation with weaker, higher order fields.
1, Scientists Confront Creationism, Laurie R. Godfrey, Ed. W.W. Norton Co., New York.
Previous threads on this subject:
Is that brief, clear and concise enough? 
I see that Exapno Mapcase got in the list of threads.
By the way, the two phenomena you’re referring to are called 1) magnetic polarity reversals and 2) secular variation of the Earth’s magnetic field.
Sure is. Magnetic North has been moving roughly northwest some 15 miles per year (sometimes more, sometimes less…just a rough average from memory) for at least the last 100 years and on a daily basis can shift about by a few miles one way or another. If you were very close to magnetic north I imagine that bouncing around would play havoc with your compass but the further south you go the less the effect would be noticeable.
This page gives some hand plots of the pole moving about.
When you’re plotting the course of a ship by magnetic compass (not a common occurance in modern navies, but a good backup skill to have should it ever be needed), the only time variation doesn’t make a difference is when magnetic north is on the same longtitude as you. Even one minute wrong can send you dozens of miles off course.
The phenomenon which leads to reversals of the Earth’s field is generally presumed to be similar to that which occurs in the Sun. It’s very well-established, both by observations and theory, that the Sun’s magnetic field reverses every 11 years, due to the flows of conductive material in the Sun. Because of the high viscosity of the corresponding fluids in the Earth (mostly molten iron, I think), the flipping of the Earth’s field is much less regular and predictable, but qualitatively, it seems to be the same.
And David, I don’t think there’s such a thing as a sextapole. The standard multipole expansion is monopole (which is never observed for magnetism), dipole, quadupole, octapole, etc. (the next one would be hexidecipole, I suppose, but moments that high are rarely used and even more rarely given a name). It’s powers of two, not even numbers.
OK. Take out all but powers of two and it’s still a magnetic field. The fluids in the earth’s core are still circulating because of convection and the rotation and it seems unlikely that they would circulate in such a fashion as to cancel all magnetic effects. I would think it much more likely that the circulations would become less organized for some reason and then reorganize with the dipole field again dominant but reversed in polarity.
A third theory, which few know and fewer support, is the magnetic field stays reletively stable while the earth’s crust slips around.
Right, I didn’t mean to imply that I disagreed. So far as I know, your post was essentially accurate (that the magnetic field would get messy, and then reform into a dipole in the other direction, without ever actually disappearing); I was just nitpicking the terminology.
This hypothetical phenomenon is called true polar wander (as opposed to apparent polar wander, which is the paleomagnetic record of a continental plate drifting across a pole). It has only been proposed for two intervals in Earth history, the Late Cretaceous (between ~83-85 Ma) and the Early Cambrian (between ~535-518 Ma); in both cases there still is a considerable about of contention.
The researchers who favor true polar wander generally point to paleomagnetic data that they interpret as revealing extremely high continental drift rates, too high to be accounted for by normal plate tectonic motion. Those who argue against true polar wander say that there are problems with the quality of the paleomagnetic data and the age constraints thereon (precise age constraints being necessary for calculating the drift rate).
More data would be useful in addressing the criticisms, but it will be particularly difficult for the Cambrian, as older rocks tend to have more problems with preserving clear, original paleomagnetic orientations.
What about religious variation?
Blessed are those that have not seen the Monopole, and still believed.
As my advisor puts it, monopoles definitely exist. There might, however, be a very small number of them, like zero.
Just rent the movie The Core. It’ll answer aaaaaaaaaaaaaall your questions, with complete scientific accuracy. 