There have been hints at nonlethal combat in both Fallout games (the Temple fight, fighting the Kahns leader for Tandi, Tandi's son, etc) but since the Fallout combat system is basically geared entirely towards lethal combat there's little benefit from trying to knock someone out instead of killing them; once you've attacked the town is probably still hostile; there's no advantage. If you could engage in brawls and such without getting the whole town or the local heat on your back, that would give you more options.
I like the idea of having influence over people, knocking out their leader or some such. I like the idea of taking captives for whatever purpose (Temporary prisoners to interrogate and release if you're good; or victims to sell, rape, torture, and experiment on if you're evil,) all with impacts on your karma, rep, and so on. As I mentioned in the thread on silencers, if the AI gets smarter to react realistically, and in this case if nonlethal combat is successfully implemented, then the game could be excellent and even more replayable.