I think another important aspect of the Eberron-pulp-1920s connection is the heroes aren't your typical stary-eyed wannabes fresh off the farm.
One of the drivers for pulp fiction was the cynicism of many American adults in the 1920s. Although the US was only in the Great War for a year, there was a large number of men who travelled overseas and saw the horrors of war. Pulps, in order to appeal to this audience, (living in cities and working in factories) had to have a more lurid nature to them. Look at most "pulp-ish" heroes and they are more jaded and cynical than heroes created from other times. Indiana Jones vs. Luke Skywalker.
Another idea which some writers have already mentioned is the advent of modern technology. If you look at many of the pulps written at this time, quite a few heroes are aviators (Terry & the Pirates) jungle explorers (Jungle Jim), and detectives (Dick Tracy). Their roles are made possible through the advances of technology and science. And each of the occupations have direct correlation to Eberron prestige classes - Extreme Explorer, Master Inquisitive & Windwrights (?).
Outside of "pulp" literature you can also see similar icons in movies - femme fatales like Marlene Dietrich and Mae West or beat down heroes like Humprey Bogart and Gary Cooper. Their attitudes - life's pushed me around and I'm gonna get what's coming to me, are popular sentiments of the time. Ever listen to the song "Buddy can you spare a dime?"
Once in khaki suits, gee, we looked swell
Full of that Yankee Doodle-de-dum.
Half a million boots went slogging through hell,
And I was the kid with the drum.
Say, don't you remember they called me Al,
It was Al all the time.
Why don't you remember, I'm your pal --
Say, buddy, can you spare a dime?
I think Eberron wants to capture some of this cynical vibe. You can play the Eberron setting without this cynicism, but I think the game will have more of a slam-bang feel of an old serial.