In the Post List, when I read an article using C-r, it is now marked as Old. (I prefer to mark things Old, myself, never having it done automatically...)
This happened in some of the 5 builds, too, so I right-clicked and "Mark Posts New" after reading, but that doesn't seem to work in 6. ("Mark Posts New" will revert a "Mark Posts Old", but not when they have been marked Old by C-r, apparently)
- mho