Another observation, if you have read a category, when a new post appears it will appear as the category is unread. If you read the new post through "New Posts" the category will still read unread. If you view the new post by clicking on the category then the category will show read. So I think the category is showing read because you access the category, not because you read the posts within. I know I have very old unread posts in several catagories, but they show read.