Thanks, now I recall and see what they are doing. Unfortunately, it's not tracking read posts at all as I got a number of threads that were marked as new because they had new posts. Clicking on the last new post should have shown me the whole thread as I'd never read it. Instead, it brought me to the first new post since my last visit.
That's ok and it's surely easier. You right, we could just set a cookie to do a similar thing. Would that be sufficient for you even though it's not truly correct?