If they have publicly promised it, and people have taken action upon it (for instance buying games that they wouldn't have bought otherwise), then if they fail carry through on it you have grounds to sue for promissory estoppel. See http://en.wikipedia.org/wiki/Estoppel#Promissory_estoppel for more on that.
Winning said lawsuit is not a sure thing. But the pledge may wind up being more binding than it would appear on the surface.
"Unless there was some situation I don't understand, we would presumably disable authentication before any event that would preclude the authentication servers from being available."
So, that's pretty clearly not a guarantee of anything. And furthermore, as another poster there said, the thing Valve employee doesn't understand is bankruptcy. Unless there is a legally binding agreement, they won't be allowed to just give away such a valuable asset. Bankruptcy doesn't mean they fire everyone, shut down everything, and burn down the buildings. It means they sell off the profitable parts and try to keep them running. And the ability to control tons of DRMed games people have installed is very valuable.
A real, legally binding promise would be something like KDE had with Trolltech. Trolltech explicitly (real contract, real lawyers, etc) guaranteed, even in bankruptcy or change of ownership, that if they stopped making open source releases, then Qt would become available under the BSD license. (This was more important back before Qt was already GPLed, as it is now)
If they have publicly promised it, and people have taken action upon it (for instance buying games that they wouldn't have bought otherwise), then if they fail carry through on it you have grounds to sue for promissory estoppel. See http://en.wikipedia.org/wiki/Estoppel#Promissory_estoppel for more on that.
Winning said lawsuit is not a sure thing. But the pledge may wind up being more binding than it would appear on the surface.