There was a discussion about the "Fallen Celestial Template" on the WotC boards shortly after Fiend Folio's release.
Someone in WotC (don't remember who) commented that a Fallen Celestial Template didn't "fit" into the "official" D&D cosmology because, as far as WotC's concerned, a Fallen Celestial is just a Fiend.
I don't know about you folks, but that's about the dumbest excuse I ever heard for dumping a concept. It makes no sense. So, if a Solar Falls, does it lose absolutely everything as a Solar and becomes either a Balor, Pit Fiend, or Ultroloth depending on its alignment?
Stupid.
I'll see if I can find the thread and post it here.