Tomorrow there is supposed to be an article on what they're doing to the cosmology, so I guess we'll find out.
Honestly, someone pointed out to me that the fiends revision is not so much anything new. "Blah, blah, blah, descriptions of stuff, devils like to trick people and demons like to eat people." But, he also pointed out to me, remember the way you had to run fiends in 2nd Edition? In our beloved and hallowed Planescape, even? Every adventurer, before stepping through the portal, would be sure to write down his cribnotes on his wrists, so as to not forget that this monster is immune to electricity, while this one takes half damage, and this one needs magic missiles forged out of cold iron, but then this other one takes half damage from normal fire so you have to use magic fire. It's no wonder everyone just started tossing acid damage. Monsters having good mechanics is what's important. The story can change from campaign to campaign. Devils are devils because of whatever your DM says.
Remember, we of Planescape are so bizarrely obsessed with our corner of the game that we constantly write story after story about how the fiends came to be, or about which plane is which. But this isn't the 4th Edition of Planescape. It's the 4th Edition of D&D. Which can be used to play Planescape. Hell, you could even (please don't kill me) play Planescape using the stuff that they're writing for the 4E "backstory." Really, Planescape is about attitude, NPCs, exploration, and philosophy more than it is about what the fiends did a million-bajillion yearses ago. Except for Shemeska's campaign, in which it is about that. But we can't all be Shemeska, as much as we wish we could be.
Can we really get so angry and demand that all D&D players play it our way? Honestly, the "Heart of Darkness" fiendish origin is cool, but it's way too involved and, frankly, planar to be stamped on every campaign in the game. If the gameworld-presence of the fiends cannot be explained to your campaign's players without involving all three major fiendish races, the Blood War, and the baernaloths, I would actually consider it inappropriate for the core game. Those things just can't be expected to be important for every D&D player. And maybe the "devils killed their god and now their Paradise is Lost and they turned all ugly-like" also doesn't work with every campaign, but the difference is that DMs who don't like this 4E story can just blot whiteout over that part and keep going. It seems like an easily swallowable, portable, light-on-creativity take on where the fiends have always come from, mostly accomplished merely by changing a few names. It's not even that wildly different in its fundamentals from the Planescape material. We've already got that stuff written, and a new edition of the game rules doesn't prevent you from playing that. Just use what you want and create the rest.
I don't like the changes they're coming up with to the planes, but those ones are mostly easily ignored...
The ideas of the implied setting suck anyways.