But I think it is an event protocol of sorts, with TODO being a subset. Subject, msg body, etc. are universal to a lot of things. For example, the difference between say email and SMS is more UI than the actual structure of the messages. The text transcript of a phone call could fit into the same bucket.
To point is, the "email" UI / UX is awful. It hasn't evolved. It might as well be a fax machine. (Goggle) Wave might have been a step in the right direction. The idea was right and solid, but Google (being Google) fucked up the launch / rollout. And now we're still stuck with the same shite email experience. And thus Graham's article is still relevant.
Note: We're all but stuck with the email protocol. The world isn't going to change en masse. That is my client experience can be improved without having to abandon the protocol. Come on. That's not risky, nor is it rocket science.