I have given this some thought. lol
One field is needed for a unix timestamp in the users table, call it U_NewPosts.
A page would be generated pulling in all threads that they have access to that are newer than this time stamp. Once a user has read all that they want to read they can upate the page and have a current timestamp set and all of the previous threads will not be displayed because they are now older than the timestamp.
If there is no value for the timestamp have it set to 24 hours ago and load the threads accordingly.
This is actually a pretty simple script to write up and I think it would be handy as I have heard of something simular if not this exact thing being requested here and there.
This has been on my list but feel free to whip it out.