I think the concept that one can touch a screen and make something happen would be alien enough to throw people off (it might simply not occur to them to try),

I think the concept that one can touch a screen and make something happen would be alien enough to throw people off (it might simply not occur to them to try), but I'm pretty sure there are at least examples of UI elements on touchscreen phones—though not entire systems—that would be fairly intuitive even to someone in the 1960s, let alone the 1980s. Modern "slide (or swipe) screen to unlock" would throw them (what the fuck does it mean to "slide" or "swipe" the screen"?) but (to pick an example I used in another post) I bet an iOS6 "slide to unlock" would be very easy for them. What do you slide? The thing that looks like it slides. Easy. How do you do it? Exactly the way you'd guess you do. I may not know what it means to "unlock" in this context, but there's not much else to do, so trying it... ta da, something happened, and there's a bunch more stuff on the screen now.