Bob,
The last post should show the last post for the thread. It was not decided to resort the threads every time a new message is posted. This is consistent with the way the bboard has always worked. I think it was decided to leave it working this way.
What is missing is a "new since last visit" feature for forums. I believe if this is implemented it should be a site-side service to allow any package to announce new content. Although it should also return pacakge specific results.
Because all the forums are in one package instance the forums package might have to sort the results further for a specific forum.
Has anyone worked on a new stuff feature with OpenACS 4?