This looks nice, but I have a small nit to pick: the expression problem concerns static type safety, so discussing it in the context of an untyped/dynamically typechecked language is a bit off.
The author mentions this presentation by Stuart Halloway on Vimeo at the end of the article - Reiterating it here - http://vimeo.com/11236603
I watched the video yesterday, and reading this article after the fact helped articulate some of what Stuart was covering in his presentation. YMMV.