I'll have to do this by examples - it's getting late. (Actually, it's getting early - I need to go to bed...) ;o)
For instance, all the stickies are "unread". I have read several of them. I think this is taken care of whenever I reread one of them. I also think this applies to all the other posts.
Bottom line is I think this issue will take care of itself within a matter of days or however long it takes the user to revisit topics of interest. That's good enough for me.
One possible improvement would be that clicking on a topic title would take you to the first unread post (like what happens when you click on "N new"). If there are several pages of posts, it is more likely that I want to pick up where I left it than at the first post.
(When I said that "that may be just a question of time" I meant that this will sort itself out as the user revisits more and more topics.)