So here's the official story (this is copied from a post in the "Descent was removed" thread):
"Hey, Folks. Here's the story.
Parallax Software still exists and still owns the copyrights to the Descent games. Under our 21-year-old agreement, Interplay has the exclusive rights to sell Descent and Descent II, and they have been doing so on Good Old Games and Steam.
The problem is that Interplay has not paid to Parallax any royalties since 2007. We've talked to them about this numerous times over the years, and finally took action this fall. We served Interplay official notice that they were in breach of the contract, and when they still failed to pay we terminated the agreement.
This means that Interplay has lost the right to sell the Descent games, which is why they came down from GOG. (We're not sure why they're still on Steam; they shouldn't be.)
Interplay does, however, still own the Descent trademark, which they are free to use or license as they see fit (such as for Descent: Underground) as long as they don't violate our copyrights.
As for whether Descent and Descent II will be available for purchase again, we hope so. We'd be very happy to work things out with Interplay.
Matt Toschlog & Mike Kulas
Parallax Software"
As an addendum, I was told that the reason it hasn't been removed from Steam is because it is published by Old Skuul and not Interplay there, and Parallax is having discussions with Old Skuul about it.