Phil and I are working out some of the details of how liveTopics should interact with the TopicExchange.

Mostly the issues are technical ones about encoding &'s and so on, but we have come across at least one philosophical class of problem:

Does the TopicExchange represent a historically accurate trail?  Or current reality.

As an example of where this is important consider deleting a post.  When you delete a post should TopicExchange "forget" all the pings made by that post (reflecting current reality) or preserve them (reflecting historical accuracy)?

So far we have come to the following conclusions, your comments are welcome:

Deleting posts is unusual (because it invalidates a permalink).  When you delete a post, you do it for a reason and would expect references to it to be deleted also.  Hence when you delete a post, it's topics will be unpinged.

If you edit the post, the topics associated with the post are re-pinged.  This will delete the old ping entry and create fresh ones.