The current Zogby survey has a lot of questions about potential Iranian and North Korean nuclear weapons, including several of the form "If [Iran|North Korea] uses a nuclear weapon against [the US|specific other country|"a US ally"|"any country"] do you favor or oppose retaliating with US nuclear weapons?" I'm marking a straight "not sure" on all of them, because I'm not, in part because I suspect it would depend on specific circumstances, but one question jumped out at me:
If North Korea uses a nuclear weapon against Japan, do you favor or oppose retaliating with U.S. nuclear weapons?