So here it is many years later and I am still thinking about dialogue. The asynchronous part is not as compelling as it used to be but it still matters. I think now my understanding of time has matured a bit so that I can see the truth in claiming that all speech is asynchronous in reference to interlocutors. We all have our own perspective and our own position. Of course the synchrony we experience is qualitatively very real and being proximal to an other in conversation or dialogue is synchronous. The asynchronous part is still vexing. I am surprised that in all these years of rapid innovation there is still very little of note happening in asynchronous technology. I remain hopeful that this will once again matter and people will build applications and tools to bridge synchronous and asynchronous experiences.
On a more current note and related to the above I speculate that indeterminacy is related to determinacy much as synchronous communication is related to asynchronous communication. As we write and record (and otherwise represent) our speech and thought it becomes real in a way that our discursive jabbering does not - or maybe I should say "real" as much as I should say substantial or manifest. This manifest speech is then frozen in time - indexed to a particular time and place but serving to "message" interlocutors at any other time and place. So we are now asynchronous.
If I were to think more about the existential and phenomenological nature of dialogue I would have to say that asynchronous dialogue is a nonsensical phrase. But I still don't really know what I am talking about.