Do pan "pores" exist, what are they, and what are their effects?
This is the microstructure of SAE 304, a steel type commonly used in pans: 



At this magnification, its "pores" look like cracks. Now see it at other magnifications (still a SAE 304, other types of steel look completely different, especially if you look at martenistic steels): 





It gets even more complicated than that, because steel structure differs between the surface and below the surface: 



As you can see, the holes are nowhere "pore" shaped. But they exist; steel is not even at the microscopic level. 

I cannot point you to more cooking-relevant sources, but this at least confirms that holes exist in the type of steel used for pans. Somebody else will have to tell us what happens to them when heated. 

Note that there is a version of the "pore" theory which insists that the "pores" are still moving at certain temperatures and static when the pan is properly preheated. I'm not sure if it's true. On the one hand, it's presented in a rouxbe video, which is generally a very good source, and I can imagine steel doing funny things on the crystal level. On the other hand, the use of the "pores" term and the lack of explanation of the underlying mechanism make me doubt it.