"I'll take a Pepsi Free, please." "You want Pepsi... FREE? What are talking about? And what's with that... anorak?"
Heh. But more seriously, in MA, it's "soda".
(And speaking of the 50's.. weren't they called "soda fountains" back then, too? And "soda straws", you know the actual ones made out of, gasp, paper?)
I'm going to have to apologize ahead of time for my relatives from IN, though, they still refer to it as "pop", although that's slowly going away I think.