I was thinking up a scenario where it might be nice to have a ping sent to you everytime someone replied to a post which has trackbacked you.  So, in essence, you could not only keep track of who has continued your conversation by trackbacking your blog, but you could also know how their conversation is going by the number of replies to their entry.  

...something like a trackback pyramid scheme.  does such a thing exist?

Somehow I think the Comeback plugin is the beginning of this concept.

But I'm not sure if both sites have to have it installed or not.