Planescape seems thoroughly part of Core, now. It was briefly its own campaign setting because the paradigm of the late '80s to middle '90s was campaign settings - I don't think the paradigm is likely to shift back, as they've seen the danger of that.
The problem seems to have been - so Ryan Dancey claimed - that people bought stuff relating to their favored campaign setting and nothing else, and avoided things relating specifically to settings they weren't into, and the market wasn't big enough for that. I think the problem can be avoided by limiting new settings to a single hardcover, preventing people from getting "addicted" to just one line. The Forgotten Realms is close to Core, and they tried, with Eberron, to create a setting into which any new supplement could be integrated. I imagine you could even fit most of the Fiendish Codex I into Eberron, excepting of course the chapter on layers. Obyriths could be minions of the Rajahs, and loumaras could come from the Dreaming Dark, while tanar'ri and their lords would all be from Sheverath (Juiblex's power level even makes sense there). So while there's a problem with FR fans not buying Eberron books, it's not as bad as Ravenloft fans not buying Dragonlance, Dark Sun, Greyhawk, FR, Planescape, or Birthright.
I think the Planar Handbook killed the chances of a true Planescape hardcover for the foreseeable future, introducing a lot of the Planescape elements but not all of them. By bringing Sigil and many of the factions into the "mainstream," it made most of what would have been in a Planescape hardcover redundant, removing any platform for presenting the rest of the setting - I mean, what would it be, Planar Handbook II? Maybe if Planar Handbook I had been a better book, there'd be a demand for that, but I doubt it. It also stripped all the factions of the context in which they made sense - when they're introduced as "clans" that most PCs will belong to, as part of a larger philosophical war that stretches across the planes, they work. As a few scattered prestige classes, they seem not intuitively "planar," leaving a lot of people I'm sure to wonder why they were in the book at all. I think the designers of the book forgot they needed to sell the material in it to the uninitiated, which required a lot more contextual effort.
So anyway, no new Planescape books, at least not until 4th edition when maybe the material can be reintroduced in context.
Maybe a Greyhawk book, though - something that could tie all the proper nouns strung through the core books and Dungeon Magazines together might be popular, if it was done well and provocatively. I don't think it would get supplements, however.
But what do I know? Maybe they don't subscribe to Ryan Dancey's philosophy at all anymore. Maybe the market's big enough to handle it.
