Yes, basically. That means if you want to create a custom table type, you only need to create an annotated function that maps messages (symbols) to behavior (functions), and voila, it all works seamlessly.
I find it amusing that your explanation pretty much sums up thousands and thousands of words that I've said. I really am too verbose. ^^;
"You were probably more intelligible to everyone else; I seem to have been uncommonly dense :) What you're describing is indeed message passing. I think I was misled by your code snippets."
I was implementing message passing by using functions + closures, so it's definitely a weird form of it... most languages implement message passing with objects + methods. So, your confusion is understandable.
Which, in fact, describes something almost completely identical to my message passing idea, except that it's more verbose in Scheme than in Arc. It even demonstrates prototypical inheritance.
The only difference is that they're using a separate function to represent each method... whereas in my examples, I've basically written the methods "inline" so there's only one function. However, I did demonstrate one way to give each method a separate function: http://arclanguage.org/item?id=14368