Riding on the coattails of the equivalent question on Spelljammer...
Is there anything happening on the Planescape front?
Both Wizards-related as well as independent updates are interesting.
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There was no mention of Planescape in any of the Gencon WotC seminars. As you're probably aware, Sigil appeared in the Dungeon Masters Guide 2, and you could cobble together some useful material from the various planar books that WotC's put out over the last two years -- but not a lot of Planescape-specific support.
Since they did mention a Shadowfell-specific box set coming next year, I suspect they'd have said something if there was a Planescape book in the works. I'm going to keep hoping for 2012, cause it's an awesome setting.
The Planewalker forums are pretty active, though!
For D&D 4e, there are three planescape-y sourcebooks:
This month, Heroes of the Elemental Chaos will be released, a supplement for elemental characters with an elemental chaos theme.
Then there's always Planewalker.
I myself prefer the great wheel cosmology to the 4e one, and therefore use the material mainly as an extension and addition to the existing planescape stuff. Recreating planescape with only 4e material is probably a futile endeavour, but it's easy to get the best of both worlds.
The only thing I could find on Planescape and fifth edition was the post 'The Many Worlds of D&D' by Mike Mearls:
"When it comes to the outer planes, we're treating Planescape as our default assumption. It's a much-beloved setting and one that's fairly easy (by design) to integrate into existing campaigns. That means the return of the Great Wheel, the Blood War, and other classic elements of the D&D cosmos. The same process for the inner planes applies to the outer planes, with our intent to add elements to the cosmos to increase storytelling opportunities and make the Wheel as flexible as possible for different settings and different DMs."
Indeed, the Player's Handbook contains a description of the Outer Planes, the Outlands, and Sigil that matches the Planescape setting (see 'Appendix C: The Planes of Existence'). However, the information presented there is very sparse and it would take a lot of imagination for a DM to base a campaign on that information. It is also presented as an option for higher level characters to travel to, not as a campaign setting in itself. The information presented there also shows some differences from the Planescape setting. For example, portals seem to be possible from any location and not just from Sigil, Feywild and Shadowfell are planes that don't exist in Planescape, and the Inner Planes are less inhospitable.
Furthermore, the Monster Manual seems to contain some specific Planescape monsters such as Modrons, Yugoloths and Slaadi. However, since I don't have any of the Planescape Monstrous Compendiums or previous Monster Manuals at hand, I am not sure how different these are from the Planescape ones and which of them have been added in 5e and which were already present in 3.5/4 edition.
I don't know if there is any material planned that actually takes place on the Outer Planes.
Planescape won't exist as it was known. As a setting it was tightly tied to 2-axis alignment wheel of AD&D, with inner, outer, elemental and the like planes. The 4th Edition cosmology just isn't compatible with that.
There is support for planes but with the changes they might as well be the Internection of Legends of Anglerre.