I'm a fool. If I'd bothered to look it up first, I would have remembered that the actor model was integral to the invention of Scheme. See Sussman & Steele, Scheme: An Interpreter for Extended Lambda Calculus (AI Memo 349), 1975 (the first link at http://library.readscheme.org/page1.html). From the acknowledgements:
This work developed out of an initial attempt to understand the actorness of actors. Steele thought he understood it, but couldn't explain it; Sussman suggested the experimental approach of actually building an "ACTORS interpreter". This interpreter attempted to intermix the use of actors and LISP lambda expressions in a clean manner. When it was completed we discovered that the "actors" and the lambda expressions were identical in implementation.
This work developed out of an initial attempt to understand the actorness of actors. Steele thought he understood it, but couldn't explain it; Sussman suggested the experimental approach of actually building an "ACTORS interpreter". This interpreter attempted to intermix the use of actors and LISP lambda expressions in a clean manner. When it was completed we discovered that the "actors" and the lambda expressions were identical in implementation.