If I read all the posts in a forum and then jump from the thread out to the part Reality Check! > Main Forums, the thread shows unread posts, as if I didn't read the one I just jumped from.
I find it easiest to read RC by going to New Posts then right clicking each new topic and opening it in another window, that way you know you haven't missed anything... This works as long as you're just on RC and don't mind having 10-20 different windows open...
I know the post marking isn't the greatest but bear with it... the next major release of this software will feature a whole new thread-marking engine!