Message passing in KO (Kay Objects) is always inherently concurrent. After all the object abstraction is "individual computers all the way down"[1], and with separate computers, I hope we agree to call the message asynchronous.
Anyway, mapping the passing of messages to synchronous lookup of a method in Smalltalk was always an optimization purely out of expediency. Highly important at the time so they could get real systems running on their "Interim Dynabooks" (aka Altos, 128K DRAM, 5.8 MHz[2]), and try them out.
However, the ability to do a full message-send was always retained in systems like Smalltalk and descendants such as Objective-C and Ruby, with the DNU hook turning the message that only exists in pieces on the call stack into a bona-fide object (Message in Smalltalk, NSInvocation in Objective-C).
And lo and behold, these hooks were and are used to implement asynchronous, remote and other forms of messaging.
So to answer your question: yes, what Ruby, Smalltalk and Objective-C do is message passing. It is probably on the extreme end of what can still be considered KO-style message passing, which to me means it is late-bound and an object representing the message for further processing can be recovered easily.
