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.
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.
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.