D&D is suffering the same problems that DC Comics ran into several years ago; decades of various disparate worlds and systems running into each other.
In some very real ways, Planescape was created simply to allow access for players to hop from one bit of hodge, to another bit of podge, but with a chance for significant adventures in between. (Note, SpellJammer attempted to do the same thing first, but I'll guess that with pressure from atmosphere intensive games such as Vampire, Mage, etc, more care was taken with PS)
And like DC Comics, the whole piecemeal structure is falling apart under it's own weight, so the GUTing (Grand Unification Theory) in 4th Edition. At least that's the lesson I'm drawing from the Reboot (that and selling more product).
It seems there are two paths which can be taken from this change:
One: Jigger the rules sufficiently for a 4e Planescape, without changing any of the underlying atmosphere, Cosmology, or religion.
Two: Attempt to transplant the favored elements from PS into the new Cosmology. Unfortunately, much would fall on the cutting room floor, such as the Modron March, the Blood War, etc. But much of the underlying tone could remain the same.
Or am I being too pollyannaish? Which tack would you prefer?
Option 3, for now: play the 2e fluff, with 3.5e mechanics and the PSCS from the fine folks here at planewalker, and worry about 4e if they ever put out interesting new fluff that makes me want to play in that meta-setting. :^)