It was still there when I got home. I could see movement in its upper abdomen, too, which I took for breathing until I saw two tiny squirming tails attached to little squirming rears. Baby possums! Now I wasn't sure if she was breathing or if the movement I saw was just them.
Lut had been doing research, and determined, among other things, that (a) possums can play possum for hours, (b) they very rarely contract rabies, and (c) they're marsupials, so they have a pouch. Hence, the babies I'd seen had gotten there via her pouch
So we were optimistic that she might be feigning death. Nonetheless, Lut dug up a wildlife sanctuary and I gave them a call, and left a message.
This morning, I still hadn't heard from the wildlife sanctuary. The possum was still in the same place, with babies now squirming inside her pouch and not otherwise visible from without.
I called animal control and left another message at the wildlife sanctuary. As Trask suspected, animal control is just going to dispose of the dead momma possum and her still-living babies. Poor baby possums. ;_;