David Gaider wrote...
There's a very good reason for why they don't enter the Fade. I'm not going to tell you what that is, but I will say that it has nothing to do with their resistance to magic. Rather the reverse: their resistance to magic comes from the fact that they don't enter the Fade -- which also allows them to mine dangerous lyrium better than other races would.
Will we ever discuss the reason they don't enter the Fade? Possibly. If that makes you pout, then sorry. 
We know that demons have some sort of connection to the Fade. Furthermore, we know that the incursion into the Fade is responsible for the Darkspawn. I would therefore presume that the constant battle between Darkspawn and Dwarves is in some way related to Dwarves not being able to enter the Fade. The piece that connects the two thoughts is missing though.
We know (ish, assuming the Codex is right on this part) that Broodmothers exist for the Dwarves, and that Genlocks are the product of Dwarven Broodmothers. So it's not simply that Dwarves are immune to the Taint. That's obviously not the case, even barring PC interactions with the Grey Wardens and Archdemon. This says to me that Dwarves at one point
could enter the Fade; after all, the Taint is from the Fade. A key piece of information we don't know is if the mages that breached the Fade into the Golden City included Dwarves or not.
At some point in time, the Dwarves stopped being able to enter the Fade. We know they can be pulled in; they're not immune to the Fade, even if they're resistant to it. At some point, an event must have occured that prevented them from entering. Per Gaider, this Fade resistance has the side effects of preventing Dreaming (capital D, as in entering the Fade) and magic in the Dwarves.
Darkspawn are spurred on by the Archdemons, who are supposedly Old Gods. However, they are not the product of them either way; the Dwarves are at constant war with them. Yet, it seems evident that the Darkspawn come from under the surface. This precludes lyrium from being a cause of any sort of Fade resistance. If lyrium caused resistance, the Darkspawn, born of the breach, would be less productive underground, not more.
My supposition is that lyrium and the Fade are directly linked. How could this be? If, and this is a big pile of if, the Golden City is indeed related to the Old Gods, then my theory is that lyrium
is the Old Gods. The common belief that the Old Gods were Dragons is in fact incorrect; after all, we see non-deific dragons. Rather, Archdemons are the product of draconic Broodmothers, much like Ogres are the product of Qunari Broodmothers, and Shrieks/Elves, Humans/Hurlocks, Dwarves/Genlocks. The "Old Gods" somehow discovered the Fade; perhaps they made it or perhaps they were the first to enter it, or perhaps not. The bodies that they left became lyrium, much like dinosaurs (and all organic matter) become oil. Hence, veins of lyrium.
That still doesn't explain Dwarves though. If exposure to lyrium
didn't make them Fade resistant (ie, the natural selection theory), there are a couple of theories. First, perhaps Elves/Humans/Qunari were involved in the breach, but Dwarves were not. This is unlikely; Dwarves are still cursed with the Taint, and perhaps bear the strongest burden.
A second theory is that Dragons interacted with the other races, but not with Dwarves, since they were undergound. We assume that races can affect one another; the Humans supposedly caused the Quickening in the Elves. The Dragons could have caused Fade affinity in the other races (Elves/Humans/maybe Qunari) but not in the the Dwarves. This seems a distinct possibility. Of course, there is a flaw in logic here... lyrium is underground.
Anyway, if any of these drunken theories are correct, I want a no-prize from Bioware... even if I don't get to know the exact details.