More on "Mark as Read"

I've been thinking more about "mark as read". I'd like this to be really simple from a readers perspective. There are primitives and cues that already exist that would make it easier to for users to explicitly indicate what they have already read and what they haven't. Note that this is different that an "already visited" cue in a link - "mark as read" must be an explicit function - perhaps even one that can be reversed by fickle readers.