I'm hardly old school since I'm 25, I was like 11 when the setting came out, but I like Planescape, for example. I shudder to think what a 4e treatment would do to it and honestly hope that what they've kludged into 4e so far is gonna be the extent of it.
I'm not much older, I'm a huge Planescape nut, and I think PS4e could be pretty solid, actually. The multiverse being "only a model" works well in Planescape, since distances and relative layout are basically meaningless (you can't walk from the Abyss to Carceri, without passing through some portals). Assuming everything still exists, somewhere, and that Sigil is the center of the campaign, you can get a lot of milage out of 4e PS. The one thing that would need to change is that "tieflings" would be re-fluffed for the setting, away from their Sword & Sorcery 4e change toward a more "urban outcast" feel for PS.
But I think the chances of seeing an official PS4e setting are pretty low. Planescape elements are being worked into the core, so I don't know what a setting book would hope to add that a "Philosophical Power" splatbook couldn't add. I also think that's why Ravenloft and Spelljammer are fairly low on the list: the stuff is core, now, and not off on it's own.
Greyhawk I wouldn't worry too much about. Dragonlance would probably be fine, too. I doubt Urban Arcana would find its way in (too modern), though I wouldn't be too surprised to hear some sort of Modern 2.0 system in the works unrelated to the next setting. Dark Sun I could see, but I worry about it, because it is dumb to make the setting fit the "everything is core" straightjacket of 4e, and, IMO, that would reduce the awesomeness of the world by a very large degree.
I don't trust Dark Sun to WotC's current plans. I could see most other settings that they would do working fine (Ravenloft would suffer the same fate, but I don't think they're likely to do a Ravenloft setting). DS would make me nervous. They would have to show me flexibility when it came to making the setting trump the rules, which they haven't. With FR, they showed that they were eager to make the rules trump the setting, in fact. If they've reversed that philosophy, it could work and be good, but Eberron kept that philosophy (even if it used a lighter touch), and nothing so far has demonstrated any sort of intention to reverse it or a questioning of their own belief in it.
I am concerned, because Dark Sun should not be a world that I would expect, for instance, any player characters with the Primal or Divine power sources. It would go a long way to ruining the feel. But 4e's philosophy to date has been to make the setting fit the rules. Which is problematic here (but it certainly isn't everywhere).
It's somwhat obvious that a 4e Dark Sun would be very different from the 2e version. The rule system is completely different and the core assumption of the games would imply change. So what ? The new version can as well be more interesting than the old(s). And if it is not, you can just plunder what you need and keep the old background.
So it wouldn't be
Dark Sun. It would be some other post apocalyptic setting that ripped off Dark Sun. If you're going to do that, you might as well drop the Dark Sun name and trot it out as something "Dark Sun-Esque" without essentially ripping off the title.
If you're going to call your setting Dark Sun, make it Dark Sun, or call it something else. It's like the whole Archon or Eladrin debacle. If you're going to use a name, it should convey the same meaning, and if it's a new thing, it should have a new name, not the same name as something else just because your crew lacks enough imagination to throw letters together without making it NounVerb AdjectiveVerber.
I'd have respect for a dark-sun-inspired setting that still worked with 4e's default assumptions. I don't have respect for someone putting ripped shorts and bronzer on an Eladrin and saying "HEY EVERYBODY IT'S DARK SUN!"