The lastest forums code includes a last_child_post field. OpenACS.org is using this in the message display to show the last reply and to sort the messages by the last reply, so the most recently replied to messages show up on top of the list.
Did you have to do anything to make that work? This code does have the last_child_post field; that's the field that's not getting updated. On several sites, fwiw - maybe it's me. :)