What are the lessons that 1940s audiences take from Casablanca?

Chumley --

Great question -- but realize that the film and the messages were
almost entirely directed to American audiences.  I lived with French
people for 2 years who were unaware of Petain's "Je Tiens Mes
Promesses Mem Celles Des Autres" - 'I Keep My Promises, Just as I Keep
the Promises of Others' from the movie.

Good review here:
http://www.filmsite.org/casa.html

For better analysis of your question I'd suggest looking at the
reviews from major American newspapers at the time.  Proquest has the
NY Times from the period online and you could pick up others from
microfilm.

Here's lookin' at you kid (with apologies for mixing Bogart metaphors),

Omnivorous-GA

Germans are bad, French are bafoons, Italians are corrupt, Moroccans
are seen and not heard.

Trusty Wikipedia provides a lot of imformation, also about the film's reception:

http://en.wikipedia.org/wiki/Casablanca_(movie)

I would disagree with the "error" about the light from the airport
shining on Rick's Cafe.  I believe the light was supposed to be from
the lighthouse that still exists with a beam that rotates 360° (not
blocked out on the land side).  True, the beam would have actually
past over the two story building.

Fantastic film!

Casablanca was made at the start of the propaganda effort launched by
Hollywood to promote the idea of getting involved in the war.  Rick,
an American, was determined to stay uninvolved.  But in the end, he
changed his stance and took an active role in fighting the Germans.

At the time, America was strongly isolationist, and had to be
convinced that the problems in Europe were worth getting involved in.

With regard to philnj's comment, in the movie, Rick, an American,
seems to be regarded as a neutral, the action is set before the US
entered WWII; but it is in fact dated 1942, so was presumably released
after entry. Either way, the message is quite clear - and for its time
very forceful - that in the face of the Nazi menace neutrality is not
an option, and even love - and traditional Hollywood endings - must be
set aside in favour of committed action.