Alright. Let's retconn Athas for WoTC:
Before anything else can begin our R&D team has to know, "What Dark Sun is." Darksun is a ripoff of Mad Max movies. Maybe "ripoff" is too strong of a word, but they are trying to emulate the flavor of Mel Gibson's setting, just like Kevin Costner did infamously with Water World. In the original Mad Max cosmology, the destruction is not due to defilement of natural resources, it is due to conflict between men after running out of resources, ultimately culminating in some kind of nuclear "'pock-ee-lypse" which left a barren landscape.
After the invasion of the Wizards of the Coast and the forces of Hasbro, there was a sweeping effort throughout the entire multiverse to incorporate material that catered to women. This has resulted in the controversial introduction of Eladrin into Dark Sun. There is a whole other thread dedicated to this controversy, so let me just say this: Accept Eladrin, WoTC wants to get you laid. Moreover, consider the model for the Dark Sun setting: Mad Max. What room for women is there in that world? It really is a male fantasy, just like other Cowboys and Indians genre works; where is the chance for the girls to Play House or Dress-Up in a world so deprived of resources that there are no houses or dresses? This was a problem grappled with in the movie franchise as well: some corporate suits somewhere said "We need more relatable female characters in this movie (so our target customers can get laid on dates)," so in MM3:BT we get Tina Turner as a villain but we also get... think about it for a minute, here's a hint: they are the Eladrin in Mad Max.
The orphans. Specifically, the head female orphan (Savannah Nix is her name apparently, according to Wikipedia). They have a small hard to reach get away out in the wilderness, but it doesn't destroy the vacant desert-y feel to the movie even though it has a waterfall, it is out past "the Dunes" and thus mostly inaccessible.
That said, the orphans ARE out of place in character: e.g. Savannah is allowed to be pretty (unlike everything else in the movie). While this allows them to be relatable for women, more than any other characters they are not products of the environment of Mad Max. They are a preserved piece of a better world (and thus represent a new hope in the movie) and can exist where they are because they got there by accident (the plane crash).
So if we were to go with Eladrin in our Illithid Dark Sun, we could use a similar narrative tool to insert them. I suggest that they were among the most recent arrivals of slave races at the time of the apocalypse. This is due to a combination of the facts that Athas is on the opposite side of the Material Plane from the origins of the Eladrin and the inherently elusive nature of Eladrin. This would also explain the small area of feywild on Athas: it is only just now starting to appear.
Now the choice to make Eladrin exclusive psions is a poor one in my opinion. Their natural proclivity for nature, elusiveness, and magic make them ideal for being the core of the Veiled Alliance. Still, we can work with this.
In Illithid Athas, the Veiled Alliance is a collection of slaves (former slaves techincally, now after the apocalypse, mostly) who were the most organized resistance to the Illithid dominion prior to the apocalypse. The big secret of the apocalypse is that the Veiled Alliance is responsible for summoning/creating the Dragon that is responsible for the apocalypse. They were trying to create a weapon against the Illithids, but miscalculated. Eladrin of course witness this destructive use of the arcane and develop their stance against it as a result. This is fortuitous for the Eladrin because they were the most responsive species to the special psionic properties of the environment and the experimentation of the Illithids (just wait until you run into an Illithid made out of one of these super-psions!).
Now I am a big fan of E6 style, so I don't like the idea of the Sorcerer Kings based on the fact that there just shouldn't be that many high level beings running around, let alone former humans. But if you wanted to keep them in Illithid Dark Sun, you could just go with their being further corruption of the Veiled Alliance guys who summoned/created the Dragon in the first place. This places defiling magic as subsequent to "preservering" magic, which does the favor of making the standard Canon magic system the historical norm on Athas, with Defiling being a locally developed recent exception, rather than vice versa. Additionally, a couple of the Sorcerer Kings could be Illithids who turned to arcane magic in this extreme circumstance, cribbing notes from the Veiled Alliance and turning to the same Outsider powers.
Most generally: Athas should be a place where the bad guys won. Because of this, I see no issue with opening Athas to the planes. The original reason for exclusion from Canon cosmologies is to explain the persistence of metal poverty on the planet. I think Outsiders can actually assist the explanation of this condition rather than hinder it. Evil Outsiders would want to either strip metal away or prevent its use. That said, it could also be the Illithids who stripped Athas. Either way, connecting Athas to evil places in larger cosmology would not disrupt the flavor of the setting: Athas is a stronghold of evil; the place where the bad guys won.
Bottom line: Illithids are a natural fit for Athas.
Post-Script: Yeah I like teh Spelljamming. I really like anything that explores the connections and relations between the large variety of beings and places in the AD&D universe.
Post-Post-Script: All I remember about Black Spine is that it made so little sense that I've blocked it from my memory. And it definitely didn't include Illithids.
Yeah. That's it exactly.
