I knew there was something familiar about Ehkahk in the excerpt for Plane Below. So I looked into Inner Planes, and found out there was indeed a Smoke Mephit Lord known as Ehkahk the Smoldering Duke.
I'm wondering if this is the first time that Ehkahk has been covered in such detail?
Nah, it's probably just another 4E retcon.
You could be right, but even if it is, it's the sort of retcon that seems perfectly reasonable to bring into Planescape using Kobold's explanation; that's it's a cover-up story to hide the fact that he's "just" a mephit. I might use that for Ehkahk myself if he ever shows up in one of my games. 
I'm mostly struck that they kept him looking exactly like a mephit, but didn't use the word "mephit" to describe him. Not even to say "this mephit-like creature might actually be the fragment of a primordial..." If he had been described as a thirty-foot tall monster that would have felt like a retcon, but keeping his 2nd edition appearance, it doesn't feel like a retcon at all.
There must not be any mephits in 4th edition.
Ehkahk, there's a nice chunk on Zerthimon, Xaos...
The more I see of 4e's fluff, the more I believe that while the high ups at WotC may not like Planescape, the people writing these books do. I'm also convinced that their trying to create a new setting, but also one that honors all of the past settings throughout D&Ds history, regardless of their taste for them.
I've only give a quick skim to this, but I feel really positive about it so far.
Mephits are in 4E, but only as familiars.
The designers said that they didn't want to have familiars with the same names as monsters to avoid confusion. I personally would rather have mephits as normal monsters.
I personally would rather not make poor world-building decisions under the excuse of "not confusing players".
Then you have a different design philosophy than the 4e designers, but both are valid philosophies by which to make a game. They put gameplay ahead of setting, and you put setting ahead of gameplay. Neither's necessarily better than the other, it's all just a matter of personal preference.
For those of you who are curious, here's a summary of the 4E mephits so far:
- All mephit familiars confer a bonus to Bluff checks, a thematically appropriate resistance and the ability to speak the elemental language of Primordial to their masters.
- The air mephit can temporarily turn invisible.
- The earth mephit can change into a stone block.
- The ice mephit can make a small area of ground slippery.
Out of these the earth mephit familiar's power seems vastly more useful.
I also don't think that having a good setting and good game play are mutually exclusive. For myself I might rename the familiars into something else and create my own mephit monsters.
Oh, they aren't mutually exclusive by any means, but it's definitely a more difficult task to work only in ways that are in favor of both. It's in many ways related to the balance between abstraction and realism. Realism is beneficial to setting, but is also often (though not necessarily always) bad for gameplay. Abstraction tends to hurt setting (by forcing more suspension of disbelief), but helps to speed up gameplay and make the system more easily graspable by new players. It's a tough job to make a system that favors both sides equally.
I'm okay with abstraction over realism. I'd much rather have a functional and entertaining game than a tabletop physics simulator. My issue is entirely with making a setting (regardless of the rules attached) simpler and "easier to understand" at the expense of a rich and complex world.
And I'm not able to just quietly accept that the other point of view is valid. We're planewalkers, people. Other beings with wrong beliefs can mess up the world we live in.
Yep! Other than that, the most detail you'll see on Ehkahk is The Inner Planes. His name originally appeared in the 1st edition Manual of the Planes, and he's mentioned in BOZ's Elemental Princes of Good article in Dragon Magazine, but there's nothing of that level of detail.
I'm amused by how much lengths the excerpt goes through to avoid using the word "mephit," however.