Planescape was mentioned in Mike Mearls' AMA on Reddit. He says it's on the "radar", but it's not entirely clear what that means. From what he said, it seems highly plausible that the setting will be released, and if not, at least some material of it will be released in some form. When? Who knows.
I have quoted the question and answer from Reddit. This particular question and answer had information about optimization that wasn't relevant to Planescape, so I ommitted those bits.
Are we going to see Planescape in some form or fashion updated to 5e? At the very least, are we going to get a Fiendish Codex-esque thing?
I like the sound of this, I can't see them bringing out Planescape until a standard setting like Forgotten Realms is released, but I'm glad it's 'on the radar'
TheLyons wrote:
IMO, "on the radar" means in a few years. Maybe. Depending on layoffs, budgets, 3rd party deals, what the Masters at Hasbro say, etc. Obviously, if it were up to big time D&D fans, and D&D developers who are big time D&D fans, there'd be a blank check written for 5e, and all this great stuff would be produced monthly. 50+ illusrators/designers, 50+ devs/writers/editors, a new office built or rented...But, that's not happening. Ever. Besides, what if D&D becomes HUGE (movies, games, everything) and then Disney buys it. 
Not that big of a deal, really...4e seemed to publish stuff really fast. Yet it took 3 years for psionics, 3 years for Dark Sun, and we never got a new setting. The last new setting was 3e's Eberron. That's a real shame. A new setting would be awesome. Of course, some old settings are cool too. Anything other than Forgettable Realms...Sorry, Greenwood fans...I just don't like it anymore. Haven't liked it (or Dragonlance, really) since I read all those TSR paperbacks in the 80's and early 90's. For me, it's Dark Sun, Planescape, Eberron, or homebrew.
seti wrote:
For comparison, here are other settings being mentioned. I copied and pasted this from enworld's cliff notes.
On Spelljammer -- "It's really a matter of time and resources, but the concept behind it is cool. SJ isn't at the front of the line, but it is in line."
On Forgotten Realms -- "We definitely want to provide a broad update on the Realms, but nothing to report yet. Sorry!"
On other settings -- "Planescape is definitely on the radar. It's a great setting on its own, plus its easy to link it to our other settings. That makes it a great direction to expand into. ... Eberron's definitely on the radar - my longest running 3e campaign was set there. A few of the rules options in the DMG speak to Eberron campaigns, plus we have the warforged designed already."
And finally, a quote from Keith Baker regarding Eberron in 5th. We can see that he has been told that Eberron will be supported (earlier statements had him say he had no idea), but he's got no new information to report than this. Looks like he's almost as in the dark as we are if his quote is to be trusted, and I firmly believe it should be. Here is a link to his FAQ
I am confident that there is going to be official 5E support for Eberron, but I don’t know when it will happen or how extensive it will be
Reius wrote: