Nick,
here are the bits I found out so far:

Adobe Sans and Serif were originally introduced with Acrobat, to serve exactly the purpose you’re describing above. While the fonts were not yet ready for the 1st release of Acrobat (instead, a MM version of Myriad could be found in there), they finally came with Acrobat version 2 (1994).
The fonts were designed by Fred Brady, but also Jim Wasco has done some work on the family.
Probably the name of a designer didn’t stick, as the fonts had to be as anonymous and as generic as possible. Also, the design concept being so constrained (having to cover everything from thin to black, condensed to wide) did probably not allow for a lot of creative involvement.

The fonts have not been modified in a long time, and still do their job (using MultipleMaster technology) in the latest version of Adobe Acrobat.
AFAIK, there is no license for them, and I guess using them outside Acrobat might be too difficult for most people anyway. :)

Thanks so much for sleuthing around for the answers, Frank. I'm assuming you got the info from some longer-standing Adobe staff members?

Though most designers groan at the sight of these fonts (after all, seeing them does usually mean something has gone wrong) I must say that the problem they solve is an interesting one. As you mentioned, they were purposefully made to be generic, but I think a lot of creativity still went into their design to get them to work as intended. Indeed, if they hadn't done a minimally reasonable job with the design, we probably wouldn't be seeing so many examples where the fonts make it out into the wild :)

As for the license, I'd be surprised if the legal team at Adobe didn't cover these fonts with some section in the EULA about bundled software or something like that.

You’re very welcome – indeed an interesting subject.
Yes, the people I have asked are Robert Slimbach and Ernie March, who both work at Adobe well over 20 years.

I've heard several times about how extreme the requirements for these fonts were. They had to be able to stand in for everything from the fattest, widest font designs to the most narrow and thin. If you've ever seen them getting used as a substitute for one of these extremes, you probably thought they looked terrible -- but the point is that they serve a dynamic range far outside the limits of what most designers consider when making an MM typeface.