Ian, thanks. This is a terrific explanation of how Twisted's threading model works. Also a terrific explanation of the Twisted culture's resistance to, but not denial of, the necessity for threads in some situations. I loved the "hurdles but no walls" comment.
Avdi, yes, Deferreds are conceptually more like a Future than a Proactor. A Proactor typically manages multiple streams of callbacks, whereas a Deferred represents an asynchronous control flow for a single call.