Before I go to far on this, I would like to clarify something that has been irritating me. Several people seem to be taking the stand the Ruin is evil, Preservation is good. This is not the case. Rather, ruin just wants to bring the world into as much chaos as possible (and will do good things if it furthers his goal.), while preservation is the force of resistance to change, the force of order (and will do evil things to do it). This same misconception happens all the time in the WOT. But anyway, with that explained, to the main body of my post.
The way I see it, this could work out two ways. Either Ruin and Preservation are actual forces/powers that are controlled by other beings (including possibly people), or they are the beings themselves and control other forces and people to get what they want.
If they are forces, one would have its highest concentration at the well of ascension. The other would have a high concentration elsewhere. After all, these forces must be very equal in nature, or one would have overpowered the other by now. The amount of control each force has over the world is determined exactly by what those who used the power did.
If they are actual beings, on the other hand, they control the world through other forces, such as the mists and hemalurgy. This makes more sense than the other theory in many ways.
I think it is most likely, however, that they are some type of blend between the two. The force itself is Ruin/Preservation, but it is conscious. They are able to affect the world based on the amount of freedom they have at a given point in time. As one becomes more free, it naturally forces the other back into its prison. I agree that it makes the most sense that preservation is also somehow bound.
But other than that, there isn't much to go on. Does anyone have the annotations where Brandon actually mentions this? It would be much appreciated.